Diffractive optical element

ABSTRACT

A diffractive optical element including a stack of first, second and third optical regions or a stack of first, second, third and fourth optical regions, a first relief pattern formed between the first and second optical regions, and a second relief patterns formed between the second and third or between the third and fourth optical regions. The first and second relief patterns have substantially identical pitch distributions and are substantially aligned in a direction of an optical axis. Depths of the first and second relief patterns are set such that a wavelength dependency of diffraction efficiency can be decreased over a wavelength range to be used. It is possible to manufacture in a simple manner at a low cost a diffractive optical element in which undesired flare and ghost are suppressed.

This is a continuation of application Ser. No. 09/672,455 filed Sep. 29,2000, now U.S. Pat. No. 6,781,756 which is a divisional of Ser. No.08/697,773 filed Aug. 29, 1996 now U.S. Pat. No. 6,157,488.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffractive optical elementcomprising a plurality of layers stacked one another to form at leastone boundary surface formed by adjacent layers made of different opticalmaterials, and a relief pattern formed in said boundary surface, andmore particularly to a diffractive optical element having a decreasedwavelength dependency of diffraction efficiency for a wide wavelengthrange.

2. Related Art Statement

The diffractive optical element of the kind mentioned above isconstituted as, for instance a diffractive lens having a convergingpower. Such a diffractive lens has the following advantages as comparedwith an ordinary refractive lens.

-   (1) The diffractive lens can be easily produce an aspherical wave,    so that aberrations can be corrected effectively.-   (2) The diffractive lens does not substantially have a thickness, so    that an optical system including such a diffractive lens can be made    compact and a freedom of design can be improved.-   (3) In the diffractive lens, a quantity corresponding to a    dispersion of the refractive lens has a negative value, and thus    chromatic aberration can be corrected effectively by a combination    of a refractive element.

The diffractive optical element having the above advantages can improvea property of an optical system as described in, for instance BinaryOptics Technology; the Theory and Design of Multi-level DiffractiveOptical Element, Gary J. Swanson, Technical Report 854, MIT LincolnLaboratory, August 1989.

As stated above, the diffractive optical element has many advantagesover the ordinary refractive optical element. However, a diffractiveefficiency of the diffractive optical element has a relatively largewavelength dependency, so that there are several problems to be solved.When the diffractive optical element is used as a lens element, it isundesired that there are formed a plurality of diffracted light rays,i.e. a plurality of focal points. Therefore, in a conventionaldiffractive lens shown in FIG. 1. a surface of a transparent substrate 1is machined to have a sawtooth relief pattern 2 such that radiant energyis constricted to a diffracted beam having a predetermined order.

When the surface of the substrate 1 is machined to have the sawtoothcross sectional configuration as illustrated in FIG. 1, a wavelength ofthe diffracted beam to which radiant energy is constricted is dependentupon a depth of recesses of the relief pattern 2 (brazed reliefpattern). Therefore, it is impossible to constrict the energy of thelight beams within a wavelength range. This phenomenon does not causeany problem for a monochromatic radiation beam such as a laser beam, butcould not be ignored for an optical system such as a camera in whichwhite light is dealt with.

When a plurality of wavelengths are used, in order to correct achromatic aberration a diffraction efficiency has to be optimized for apredetermined single wavelength. Then a diffraction efficiency isdecreased for wavelengths other than said predetermined wavelength.Particularly, when the diffractive optical element is applied to animage pick-up optical system for picking-up a visible light image, theremight be produced a variation in color and flare due to light beams ofundesired orders.

FIG. 2 is a graph showing a wavelength dependency of a first orderdiffraction efficiency of the known diffractive optical element havingthe substrate 1 made of BK7 and the relief pattern 2 having such a depththat a first order diffraction efficiency becomes 100% for a wavelengthλ=520 nm. As can be seen from the graph of FIG. 2, within a visiblewavelength range from 400 nm to 700 nm, a diffractive efficiency becomesmaximum at a wavelength of 520 nm and becomes smaller as a wavelengthdeparts from the optimum wavelength of 520 nm. Particularly, adiffractive efficiency is decreased largely when a wavelength becomesshorter than 520 nm. Such a decrease in a diffractive efficiency forwavelengths other than the predetermined wavelength might causeundesired effect upon an optical system due to an increase in lightbeams of undesired orders. This apparently affects the function of theoptical system including the diffractive optical element.

The relief pattern 2 having the sawtooth cross sectional shape as shownin FIG. 1 may be represented by a phase shift function φ(x) illustratedin FIG. 3. This function φ(x) characterizes a wave front modulation bythe relief pattern, and can be expressed by a periodic functioncorresponding to the sawtooth configuration of the relief pattern. Anm-order diffraction efficiency ηm of the relief pattern expressed by thephase shift function φ(x) may be given as follows:

$\begin{matrix}{\eta_{m} = \left\{ \frac{{\sin\left( {m - a} \right)}\;\pi}{\left( {m - a} \right)\;\pi} \right\}^{2}} & (1)\end{matrix}$wherein a is a amplitude of variation and will be expressed as a phaseamplitude hereinafter.

In the equation (1), the phase amplitude a may be defined by a thefollowing equation:

$\begin{matrix}{a = \frac{\left( {n - 1} \right)\mspace{11mu} d}{\lambda}} & (2)\end{matrix}$wherein n is a refractive index of the substrate 1, d is a depth of therecess, and (λ) is a wavelength of light to be used. It should be notedthat a refractive index of an air is assumed to be a unit. When a depthd₀ is optimized such that a diffraction efficiency of m₀ order for awavelength λ₀ becomes 100%, the depth d₀ may be expressed as follows:

$\begin{matrix}{d_{0} = \frac{m_{0}\lambda_{0}}{{n\left( \lambda_{0} \right)} - 1}} & (3)\end{matrix}$Then, the phase amplitude a (λ) may be represented by the followingequation (4).

$\begin{matrix}{{a(\lambda)} = {m_{0} \cdot \frac{{n(\lambda)} - 1}{{n\left( \lambda_{0} \right)} - 1} \cdot \frac{\lambda_{0}}{\lambda}}} & (4)\end{matrix}$

The above equation (4) means that for a given depth d₀ the phaseamplitude is dependent upon the wavelength. Due to this dependency ofthe phase amplitude a upon the wavelength, the wavelength dependency ofthe diffraction efficiency occurs as shown in FIG. 2.

The inventor has investigated the mechanism of the wavelength dependencyof diffraction efficiency and has proposed a novel relief typediffractive optical element in which the wavelength dependency ofdiffraction efficiency is reduced. This diffractive optical element isdisclosed in a co-pending U.S. patent application Ser. No. 08/522,292filed on Sep. 7, 1995. This diffractive optical element is illustratedin FIG. 4 of the instant application. In this optical element, a firstoptical layer 3 made of an optical material having a high refractiveindex and a low dispersion and a second optical layer 4 made of anoptical material having a low refractive index and a high dispersion arestacked such that a relief pattern 5 is formed in a boundary surface ofthese layers. It should be noted that the dispersion means a dispersionof a refractive index for a variation of a wavelength. When the reliefpattern 5 is shaped into a sawtooth configuration, a phase amplitude a(λ) may be given by the following equation (5) upon optimizing therecess depth in such a manner that the diffraction efficiency of m₀order for a wavelength λ₀n₂(λ) becomes 100%.

$\begin{matrix}{{a(\lambda)} = {m_{0} \cdot \frac{{n_{1}(\lambda)} - {n_{2}(\lambda)}}{{n_{1}\left( \lambda_{0} \right)} - {n_{2}\left( \lambda_{0} \right)}} \cdot \frac{\lambda_{0}}{\lambda}}} & (5)\end{matrix}$wherein n₀(λ) is a refractive index of the first optical layer 3 andn₂(λ) is a refractive index of the second optical layer 4.

In the above equation (5), when n₁(λ)>n₂(λ) is satisfied as shown inFIG. 5 for a whole wavelength range to be used, a difference in arefractive index in a numerator becomes increased in accordance with anincrease in the wavelength λ, and thus a variation of the wavelength λin a denominator is canceled out. Therefore, the wavelength dependencyof phase amplitude is reduced and thus the wavelength dependency ofdiffraction efficiency can be also reduced.

However, in practice, many optical materials having a large refractiveindex have also a large dispersion. Therefore, it is very difficult tofind out a suitable combination of the first and second optical layers 3and 4. For instance, there have been various kinds of optical materialswhich can be used for a visible wavelength range, but in thesematerials, a dispersion is increased in accordance with in increase in arefractive index. Moreover, almost all optical materials for a visiblewavelength range are made of optical glasses which could not be machinedeasily. Therefore, when these optical glasses are combined, it isdifficult to form a desired relief pattern in a boundary surfacetherebetween. In order to mitigate such a drawback, at least one of theoptical layers may be made of optical plastic material which can bemachined relatively easily. However, in such a case, plastic materialsare limited and thus it is difficult to select a suitable combination ofoptical materials of the two optical layers 3 and 4. Particularly, it isquite difficult to improve the wavelength dependency of diffractionefficiency by a combination of two plastic materials.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a novel and usefuldiffractive optical element, which can be manufactured easily and cansuppress undesired flare and ghost by decreasing a wavelength dependencyof diffraction efficiency.

According to a first aspect of the invention, a diffractive opticalelement comprises:

-   a first optical region made of a first optical material which is    substantially transparent to light within a wavelength range to be    used;-   a second optical region made of a second optical material which is    substantially transparent to said light but is different from said    first optical material;-   a third optical region made of a third optical material which is    transparent to said light but is different from said second optical    material, said first, second and third optical regions being    arranged to be brought into contact with each other or being    arranged close to each other;-   a first relief pattern formed in a boundary surface between said    first and second optical regions and having a first pitch    distribution and a first depth; and-   a second relief pattern formed in a boundary surface between said    second and third optical regions and having a second pitch    distribution which is substantially identical with said first pitch    distribution of the first relief pattern and a second depth which is    different from said first depth of the first relief pattern, said    first and second relief patterns being substantially aligned in a    direction of an optical axis of the diffractive optical element.

According to a second aspect of the invention, a diffractive opticalelement comprises:

-   a first optical region made of a first optical material which    reflects light within a wavelength range to be used;-   a second optical region made of a second optical material which is    substantially transparent to said light;-   a third optical region made of a third optical material which is    substantially transparent to said light but is different from said    second optical material, said first, second and third optical    regions being arranged to be brought into contact with each other or    being arranged close to each other;-   a first relief pattern formed in a boundary surface between said    first and second optical regions and having a first pitch    distribution and a first depth; and-   a second relief pattern formed in a boundary surface between said    second and third optical regions and having a second pitch    distribution which is substantially identical with said first pitch    distribution of the first relief pattern and a second depth which is    different from said first depth of the first relief pattern, said    first and second relief patterns being substantially aligned in a    direction of an optical axis of the diffractive optical element.

According to a third aspect of the invention, a diffractive opticalelement comprises:

-   a first optical region made of a first optical material which is    substantially transparent to light within a wavelength range to be    used and has a refractive index n₁;-   a second optical region made of a second optical material which is    substantially transparent to said light but is different from said    first optical material and has a refractive index n₂;-   a third optical region made of a third optical material which is    transparent to said light but is different from said first and    second optical materials and has a refractive index n₃, said first,    second and third optical regions being arranged to be brought into    contact with each other or being arranged close to each other;-   a first relief pattern formed in a boundary surface between said    first and second optical regions and having a first pitch    distribution and a depth d₁; and-   a second relief pattern formed in a boundary surface between said    second and third optical regions and having a second pitch    distribution which is substantially identical with said first pitch    distribution of the first relief pattern and a second depth d₂, said    first and second relief patterns being substantially aligned in a    direction of an optical axis of the diffractive optical element,    wherein when a ratio of the depth of the second relief pattern to    the depth of the second relief pattern is α(=d₂/d₁), a wavelength of    the light to be used is λ, a shortest wavelength of a wavelength    region of the light to be used is λ₁, and a longest wavelength of    the light is λ₂, the following condition is satisfied:    |ΔN(λ₂)|>|ΔN(λ₁)|>0;λ₂>λ₁  (6)    wherein    ΔN(λ)={n ₁(λ)−n ₂(λ)}+α{n ₂(λ)−n ₃(λ)}  (7)

According to a fourth aspect of the invention, a diffractive opticalelement comprises:

-   a first optical region made of a first optical material which is    substantially transparent to light within a wavelength range to be    used;-   a second optical region made of a second optical material which is    substantially transparent to said light but is different from said    first optical material;-   a third optical region made of a third optical material which is    substantially transparent to said light;-   a fourth optical region made of a fourth optical material which is    transparent to said light but is different from said third optical    material, said first, second, third and fourth optical regions being    arranged to be brought into contact with each other or being    arranged close to each other;-   a first relief pattern formed in a boundary surface between said    first and second optical regions and having a first pitch    distribution and a first depth; and-   a second relief pattern formed in a boundary surface between said    third and fourth optical regions and having a second pitch    distribution which is substantially identical with said first pitch    distribution of the first relief pattern and a second depth which is    different from said first depth of the first relief pattern, said    first and second relief patterns being aligned in a direction of an    optical axis of the diffractive optical element.

According to a fifth aspect of the invention, a diffractive opticalelement comprises:

-   a first optical region made of a first optical material which    reflects light within a wavelength range to be used;-   a second optical region made of a second optical material which is    substantially transparent to said light;-   a third optical region made of a third optical material which is    substantially transparent to said light;-   a fourth optical region made of a fourth optical material which is    substantially transparent to said light but is different from said    third optical material, said first, second, third and fourth optical    regions being arranged to be brought into contact with each other or    being arranged close to each other;-   a first relief pattern formed in a boundary surface between said    first and second optical regions and having a first pitch    distribution and a first depth; and-   a second relief pattern formed in a boundary surface between said    third and fourth optical regions and having a second pitch    distribution which is substantially identical with said first pitch    distribution of the first relief pattern and a second depth which is    different from said first depth of the first relief pattern, said    first and second relief patterns being substantially aligned in a    direction of an optical axis of the diffractive optical element.

According to a sixth aspect of the invention, a diffractive opticalelement comprises:

-   a first optical region made of a first optical material which is    substantially transparent to light within a wavelength range to be    used and has a refractive index n₁;-   a second optical region made of a second optical material which is    substantially transparent to said light but is different from said    first optical material and has a refractive index n₂;-   a third optical region made of a third optical material which is    substantially transparent to said light and has a refractive index    n₃;-   a fourth optical region made of a fourth optical material which is    transparent to said light but is different from said third optical    material and has a refractive index n₄, said first, second, third    and fourth optical regions being arranged to be brought into contact    with each other or being arranged close to each other;-   a first relief pattern formed in a boundary surface between said    first and second optical regions and having a first pitch    distribution and a first depth d₁; and-   a second relief pattern formed in a boundary surface between said    third and fourth optical regions and having a second pitch    distribution which is substantially identical with said first pitch    distribution of the first relief pattern and a second depth d₂, said    first and second relief patterns being substantially aligned in a    direction of an optical axis of the diffractive optical element;    wherein a ratio of the first depth to the depth of the second depth    is α(=d₂/d₁), a wavelength of the light to be used is λ, a shortest    wavelength of a wavelength region of the light to be used is λ₁, and    a longest wavelength is λ₂, the following condition is satisfied:    ΔN(λ₂)|>|ΔN(λ₁)|>0;λ₂>λ₁  (8)    wherein    ΔN(λ)={n ₁(λ)−n ₂(λ)}+α{n ₃(λ)−n ₄(λ)}  (9)

It should be noted that according to the invention, the first and secondrelief patterns are substantially aligned in the direction of theoptical axis of the diffractive optical element. This includes twocases; in a first case top and bottom portions of the first reliefpatterns are substantially aligned with top and bottom portions of thesecond relief pattern, respectively viewed in the direction of theoptical axis, and in a second case, top an bottom portions of the firstrelief pattern are substantially aligned with bottom and top portions ofthe second relief pattern, respectively viewed in the direction of theoptical axis. In the first case, it may be stated that the first andsecond relief patterns can be considered to be arranged in the samedirection, whilst in the second case, it may be said that the secondrelief pattern is opposed to the first relief pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a known diffractive opticalelement;

FIG. 2 is a graph denoting a wavelength dependency of diffractionefficiency of the known element shown in FIG. 1;

FIG. 3 is a graph expressing a phase shift function of the diffractiveoptical element of FIG. 1;

FIG. 4 is a cross sectional view illustrating a diffractive opticalelement disclosed in a co-pending U.S. patent application;

FIG. 5 is a graph denoting wavelength dependencies of refractive indicesof the diffractive optical element shown in FIG. 4;

FIG. 6 is a schematic cross sectional view depicting a principalstructure of the diffractive optical element according to a first aspectof the invention;

FIG. 7 is a schematic cross sectional view depicting a principalstructure of the diffractive optical element according to a secondaspect of the invention;

FIG. 8 is a cross sectional view illustrating a first embodiment of thediffractive optical element according to the invention;

FIG. 9 is a graph representing wavelength dependencies of differences inrefractive index between successive optical regions of the firstembodiment;

FIG. 10 is a graph showing wavelength dependencies of phase amplitude ofthe first embodiment and a known diffractive optical element;

FIG. 11 is a graph denoting wavelength dependencies of diffractionefficiency of the first embodiment and the known diffractive opticalelement;

FIG. 12 is a cross sectional view showing a modification of the firstembodiment shown in FIG. 8;

FIG. 13 is a cross sectional view depicting a second embodiment of thediffractive optical element according to the invention;

FIG. 14 is a graph representing wavelength dependencies of differencesin refractive index between successive optical regions of the secondembodiment;

FIG. 15 is a graph representing wavelength dependencies of diffractionefficiency of the second embodiment and a known diffractive opticalelement;

FIG. 16 is a cross sectional view showing a third embodiment of thediffractive optical element according to the invention;

FIG. 17 is a graph representing wavelength dependencies of diffractionefficiency of the third embodiment and a known diffractive opticalelement;

FIG. 18 is a cross sectional view illustrating a fourth embodiment ofthe diffractive optical element according to the invention;

FIG. 19 is a graph expressing wavelength dependencies of diffractionefficiency of the fourth embodiment and a known diffractive opticalelement;

FIG. 20 is a cross sectional view depicting a fifth embodiment of thediffractive optical element according to the invention;

FIG. 21 is a graph denoting wavelength dependencies of diffractionefficiency of the fifth embodiment and a known diffractive opticalelement;

FIG. 22 is a schematic view illustrating an image pick-up devicecomprising the diffractive optical element according to the invention;

FIG. 23 is a schematic view showing a view finder comprising thediffractive optical element according to the invention;

FIG. 24 is a schematic cross sectional view depicting a principalstructure of the diffractive optical element according to a fourthaspect of the invention;

FIG. 25 is a schematic cross sectional view depicting a principalstructure of the diffractive optical element according to a fifth aspectof the invention;

FIG. 26 is a cross sectional view illustrating a sixth embodiment of thediffractive optical element according to the invention;

FIG. 27 is a graph representing wavelength dependencies of differencesin refractive index between successive optical regions of the sixthembodiment;

FIG. 28 is a graph showing wavelength dependencies of phase amplitude ofthe sixth embodiment and a known diffractive optical element;

FIG. 29 is a graph expressing wavelength dependencies of diffractionefficiency of the sixth embodiment and the known diffractive opticalelement;

FIG. 30 is a cross sectional view showing a modification of the sixthembodiment shown in FIG. 8;

FIG. 31 is a cross sectional view depicting a second embodiment of thediffractive optical element according to the invention;

FIG. 32 is a graph representing wavelength dependencies of differencesin refractive index between successive optical regions of the seventhembodiment;

FIG. 33 is a graph denoting wavelength dependencies of diffractionefficiency of the seventh embodiment and a known diffractive opticalelement;

FIG. 34 is a cross sectional view showing a eighth embodiment of thediffractive optical element according to the invention;

FIG. 35 is a graph representing wavelength dependencies of diffractionefficiency of the eighth embodiment and a known diffractive opticalelement;

FIG. 36 is a cross sectional view showing a ninth embodiment of thediffractive optical element according to the invention;

FIG. 37 is a graph expressing wavelength dependencies of diffractionefficiency of the ninth embodiment and a known diffractive opticalelement;

FIG. 38 is a cross sectional view showing a tenth embodiment of thediffractive optical element according to the invention;

FIG. 39 is a cross sectional view depicting a modification of the tenthembodiment of the diffractive optical element according to theinvention; and

FIG. 40 is a graph expressing wavelength dependencies of diffractionefficiency of the tenth embodiment and a known diffractive opticalelement.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 is a schematic cross sectional view illustrating a portion of anembodiment of the diffractive optical element according to the firstaspect of the invention. The diffractive optical element comprisesfirst, second and third optical regions 11, 12 and 13, which are stackedone on the other successively, a first relief pattern 21 formed in aboundary surface between the first and second regions 11 and 12, and asecond relief pattern 22 formed in a boundary surface between the secondand third regions 12 and 13. The first, second and third optical regions11, 12 and 13 are made of different optical materials which aresubstantially transparent to light within a wavelength range to be used,but have different refractive indices n₁, n₂ and n₃, respectively.

The first and second relief patterns 21 and 22 are formed to have asawtooth cross sectional configuration having the same pitchdistribution. A depth of the first sawtooth relief pattern 21 is d₁, anda depth of the second sawtooth relief pattern 22 is d₂, and a distancebetween a top of the first relief pattern 23 and a bottom of the secondrelief pattern is d₃.

In the diffractive optical element shown in FIG. 6, light is madeincident upon the diffractive optical element along an optical axisthereof which is a vertical direction in a plane of the drawing of FIG.6. Then, the incident light is subjected to a phase modulation by thesecond and first relief patterns 21 and 22. In this case, a phaseamplitude a₁(λ)′ of the first relief pattern 21 may be expressed asfollows:

$\begin{matrix}{{{a_{1}(\lambda)} = {\frac{\Delta\;{n_{1}(\lambda)}}{\lambda}d_{1}}};{{\Delta\;{n_{1}(\lambda)}} = {{n_{1}(\lambda)} - {n_{2}(\lambda)}}}} & (10)\end{matrix}$

A phase amplitude a₂(λ) of the second relief pattern 22 may berepresented in the following manner:

$\begin{matrix}{{{a_{2}(\lambda)} = {\frac{\Delta\;{n_{2}(\lambda)}}{\lambda}d_{2}}};{{\Delta\;{n_{2}(\lambda)}} = {{n_{2}(\lambda)} - {n_{3}(\lambda)}}}} & (11)\end{matrix}$

Now it is assumed that the first and second relief patterns 21 and 22are constructed into a single integral body and the light is modulatedsubstantially at the same time. Then, a total phase amplitude a(λ) whichcharacterizes the phase shift function may be expressed by the followingequation:

$\begin{matrix}{{a(\lambda)} = {{{a_{1}(\lambda)} + {a_{2}(\lambda)}} = {\frac{{\Delta\;{n_{1}(\lambda)}} + {\alpha\;\Delta\;{n_{2}(\lambda)}}}{\lambda}d_{1}}}} & (12)\end{matrix}$

For a specific wavelength λ₀, if a diffraction efficiency of m₀ orderbecomes 100%, a(λ₀)=m₀ is obtained. Therefore, when the depths of thefirst and second relief patterns 21 and 22 are optimized in such amanner that a diffraction efficiency of m₀ order becomes 100%, thefollowing equation (13) may be obtained:

$\begin{matrix}{{a(\lambda)} = {m_{0}{\frac{{\Delta\;{n_{1}(\lambda)}} + {\alpha\;\Delta\;{n_{2}(\lambda)}}}{{\Delta\;{n_{1}\left( \lambda_{0} \right)}} + {\alpha\;\Delta\;{n_{2}\left( \lambda_{0} \right)}}} \cdot \frac{\lambda_{0}}{\lambda}}}} & (13)\end{matrix}$wherein α is a value defined by a ratio of the depth d₂ of the secondrelief pattern 22 to the depth d₁ of the first relief pattern 21. Thatis α=d₂/d₁.

As expressed by the equation (12), the phase amplitude a(λ) of thediffractive optical element according to the first aspect of theinvention is given by a sum of the phase amplitude a₁(λ)of the firstrelief pattern 21 and the phase amplitude a₂(λ) of the second reliefpattern 22, and a wavelength dependency of the phase amplitude a(λ) ofthe diffractive optical element is dependent upon the parameter α. Here,the parameter α may be determined at will in regardless of theoptimization of the diffraction efficiency for the specific wavelengthλ₀.

In the diffractive optical element illustrated in FIG. 6, the first,second and third optical regions 11, 12 and 13 are made of differentoptical materials, and the differences in refractive index Δn₁ and Δn₁have different wavelength dependencies from each other. Therefore, bychanging the parameter α, the wavelength dependency of the phaseamplitude defined by the equation (13) can be varied.

As explained above, according to the first aspect of the invention, onlythe wavelength dependency of the diffraction efficiency at the specificwavelength λ₀ can be independently adjusted or controlled in a favorablemanner by suitably setting the ratio of the depth of the second reliefpattern to the depth of the first relief pattern, i.e. the parameter α,while a diffractive efficiency at the specific wavelength λ₀ can bemaintained to be optimum. In general, it is possible to optimize thewavelength dependency of the diffractive efficiency by making the depthsdifferent from each other (α≠1). However, according to the invention, itis also possible to optimize the wavelength dependency of thediffraction efficiency by suitably combining the optical materials ofthe first, second and third optical regions even under a condition thatα=1.

FIG. 7 is a schematic cross sectional view illustrating the diffractiveoptical element according to the second aspect of the invention. Thediffractive optical element comprises first, second and third opticalregions 14, 15 and 16, which are stacked one on the other successively,a first relief pattern 23 formed in a boundary surface between the firstand second regions 14 and 15, and a second relief pattern 24 formed in aboundary surface between the second and third regions 15 and 16. Thefirst optical region 14 is made of a material which reflects lightwithin a wavelength range to be used. The second and third opticalregions 15 and 16 are made of different optical materials which aresubstantially transparent to light within the wavelength range to beused, but have different refractive indices n₂ and n₃, respectively.

The first and second relief patterns 21 and 22 are formed to have asawtooth cross sectional configuration having the same pitchdistribution like as the diffractive optical element illustrated in FIG.6. A depth of the first sawtooth relief pattern is d₁, and a depth ofthe second sawtooth relief pattern is d₂, and a distance between a topof the first relief pattern 23 and a bottom of the second relief patternis d₃.

In FIG. 7, light is made incident upon the diffractive optical elementfrom the side of the third optical region 16 and is subjected to thephase modulation by the second and first relief patterns 24 and 23 inthis order. The first relief pattern 23 is formed on the surface of thefirst optical region 14 made of the material which reflects the light,and thus the light is reflected by the first relief pattern 23. In thismanner, the diffractive optical element of this embodiment serves as areflection type diffractive optical element.

In the present diffractive optical element, it is also assumed that thefirst and second relief patterns 23 and 24 are constructed as a singleintegral body, so that the light impinging upon the diffractive opticalelement is modulated substantially simultaneously. Then, a phaseamplitude of the diffractive optical element may be expressed by thefollowing equation (14) when the depths of the first and second reliefpatterns 23 and 24 are set in such a manner that the diffractionefficiency of m₀ order for a specific wavelength λ₀ becomes 100%.

$\begin{matrix}{{a(\lambda)} = {m_{0} \cdot \frac{{- {n_{2}(\lambda)}} + {\alpha\left\{ {{n_{2}(\lambda)} - {n_{3}(\lambda)}} \right\}}}{{- {n_{2}\left( \lambda_{0} \right)}} + {\alpha\left\{ {{n_{2}\left( \lambda_{0} \right)} - {n_{3}\left( \lambda_{0} \right)}} \right\}}} \cdot \frac{\lambda_{0}}{\lambda}}} & (14)\end{matrix}$

This equation (14) corresponds to the equation (13) of the diffractiveoptical element shown in FIG. 6 and the parameter α is a ratio of thedepth d₂ of the second relief pattern 24 to the depth d₁ of the firstrelief pattern 23.

The equation (14) may be obtained by setting the refractive index of thefirst optical region 14 in the equation (13) of the first embodimentshown in FIG. 6 to zero. This means that also in the present diffractiveoptical element, the phase amplitude can be expressed by the parameter αwhich may take an arbitrary value. Therefore, also in the presentdiffractive optical element, by suitably setting the parameter α, it ispossible to control only the wavelength dependency of diffractionefficiency, while keeping the diffraction efficiency for the specificwavelength optimum like as the first embodiment.

In the first aspect of the invention, in order to further reduce thewavelength dependency of diffraction efficiency, it is necessary todecrease the wavelength dependency of phase amplitude a(λ) in theequation (13). In this equation, the wavelength dependency of phaseamplitude a(λ) is determined by the two terms including the differencesin refractive index Δn₁(λ) in the numerator and λ in the denominator.Therefore, in order to reduce this wavelength dependency, it ispreferable to select the optical materials and said ratio of the depthsin such a manner that an absolute value of a sum of the two differencesin refractive index ΔN(λ) is increased in accordance with an increase ina wavelength λ. Said absolute value of a sum of the two differences inrefractive index ΔN(λ) can be expressed by the following equation (15):ΔN(λ)=|Δn ₁(λ)+αΔn ₂(λ)|  (15)

This is the third aspect of the present invention.

According to the third aspect of the invention, the numerator anddenominator give opposite functions upon the wavelength dependency ofdiffraction efficiency, and thus it is possible to realize thediffractive optical element having a further reduced wavelengthdependency of diffraction efficiency. The equation (13) is defined forthe structure according to the first aspect of the invention, but theequation (14) may be obtained by setting n₁(λ)=0 in the equation (13),so that the explanation about the equation (15) may be also applied tothe second aspect of the invention.

In case of actually selecting optical materials for the diffractiveoptical element, there are many optical materials, in which an absolutevalue of the difference in refractive index Δn(λ) becomes decreased inaccordance with an increase in a wavelength λ. This does not result inthe desired wavelength dependency. That is to say, many availableoptical materials have high refractive index and high dispersion or lowrefractive index and low dispersion. In the first and second aspects ofthe invention, it is effective to setting a sign, i.e. positive ornegative of the ratio α of depths of the grooves of the relief patternssuch that the two terms including the difference in refractive index arecanceled out. Then, the wavelength dependency opposite to the desiredproperty can be canceled out. In this manner, by combining opticalmaterials having high refractive index and high dispersion with opticalmaterials having low refractive index and low dispersion, the wavelengthdependency of diffraction efficiency can be further reduced. It shouldbe noted that said combination can be easily realized.

The sign of the ratio α can be determined by a manner of arranging thefirst and second relief patterns. When the first and second reliefpatterns are arranged such that tops and bottoms of the first patternare aligned with tops and bottoms of the second pattern in the directionof the optical axis as illustrated in FIG. 8, the ratio α is positive.When the first and second relief patterns are arranged such that topsand bottoms of the first pattern are aligned with bottoms and tops ofthe second pattern in the direction of the optical axis as shown in FIG.13, it is possible to obtain a negative ratio α.

In the first and second aspects of the invention, the third opticalregions 13 and 16 may be an atmosphere which surrounds the diffractiveoptical element. In a usual condition, the diffractive optical elementis placed in the air, and then the third optical regions 13 and 16 areconstituted by the air. In this case, a difference in refractive indexbetween the air and the transparent solid state optical material of thesecond optical region can be large. Therefore, a depth of grooves of thesecond relief pattern can be thin, and thus it is possible to realizethe diffractive optical element having a superior property.

In general, the diffractive optical element may be classified into athick type and a thin type. In case realizing a diffractive opticalelement for an image focusing optical system using light rays within awavelength range, it is preferable to use the thin type one which has arelatively low incident angle dependency as well as a relatively lowwavelength dependency. It has been well known to use a parameter Qcharacterizing a thickness of the diffractive optical element, saidparameter being defined as follows:

$\begin{matrix}{Q = \frac{2{\pi\lambda}\; D}{n_{0}T^{2}}} & (16)\end{matrix}$

In general, when Q<1, a diffractive optical element is classified intothe thin type. In the above equation (16), T is a pitch of a periodicstructure of a relief pattern and n₀ is an average refractive index ofthe periodic structure. In the present invention, it is also preferableto construct the diffractive optical element to satisfy the conditionQ<1.

From the equation (16), it can be understood that the parameter Qdenoting the thickness of the diffractive optical element is dependentupon the wavelength. However, in order to keep a uniformity of thediffraction efficiency over a whole wavelength range to be used, it issufficient that the above condition Q<1 is satisfied for a centerwavelength within the whole wavelength range. Therefore, also in thepresent invention, it is preferable to construct the diffractive opticalelement such that the condition Q<1 is satisfied for a centerwavelength. When the diffractive optical element is used for a visiblelight wavelength, the center wavelength may be set to a value within arange from 480 nm to 550 nm. Of course, it is much more preferable toconstruct the diffractive optical element to satisfy the condition Q<1for the whole wavelength range.

The inventor has confirmed that when Q<0.1, the relief type diffractiveoptical element can have characteristics of the thin type in anefficient manner. Therefore, according to the invention, it ispreferable to construct the periodic structure to satisfy the conditionQ<0.1.

In the embodiment shown in FIG. 6, the depth D and average refractiveindex n₀ of the periodic structure may be expressed as follows:D=d ₁ +d ₂ +d ₃  (17)

$\begin{matrix}{n_{0} = \frac{{d_{1}\frac{n_{1} + n_{2}}{2}} + {d_{2}\frac{n_{2} + n_{3}}{2}} + {d_{3}n_{2}}}{d_{1} + d_{2} + d_{3}}} & (18)\end{matrix}$

In the embodiment shown in FIG. 7, the depth D and average refractiveindex n₀ of the periodic structure may be expressed as follows:D=2(d ₁ +d ₂ +d ₃)  (19)

$\begin{matrix}{n_{0} = \frac{{d_{1}\frac{n_{2}}{2}} + {d_{2}\frac{n_{2} + n_{3}}{2}} + {d_{3}n_{2}}}{d_{1} + d_{2} + d_{3}}} & (20)\end{matrix}$

It should be noted that the diffractive optical element according to theinvention is particularly suitable for a wavelength wider than a givenamount. In conventional diffractive optical element in which thediffraction efficiency is optimized for an arbitrary wavelength λ, awidth of a wavelength range in which a variation of the diffractionefficiency can be neglected is about 5%. Therefore, the diffractiveoptical element according to the invention can be advantageously usedfor a wavelength range which is wider than 5% of a center wavelength λ.

The above explanation is described for a case in which the first, secondand third optical regions are brought into contact with each other.However, this may be also applied to a diffractive optical element inwhich said optical regions are brought closer to each other by means ofcementing layers provided between adjacent optical regions.

The diffractive optical element according to the invention may be usedfor general optical systems in which a plurality of wavelengths within awavelength range are used, but the diffractive optical element accordingto the invention can be particularly advantageously used in a visiblelight image focusing optical system.

FIG. 22 is a schematic view showing a camera in which the diffractiveoptical element according to the invention is utilized as an imagepick-up lens. In FIG. 22, an image pick-up optical system 60 comprises arefractive type lens 51 and a diffractive optical element 41 accordingto the invention. The image pick-up optical system 60 forms a visibleimage of an object on an image pick-up device 61 such as CCD. Thediffractive optical element 41 according to the invention has a veryhigh diffractive efficiency over a whole visible wavelength range, andtherefore it is possible to effectively suppress undesired flare andghost in case of picking-up a color image.

FIG. 23 is a schematic view illustrating another optical system in whichthe diffractive optical element according to the invention is installed.In the present embodiment, the optical 1 system is constructed as a viewfinder of a camera or an ocular optical system of a microscope. In FIG.23, an objective lens 53 forms a real image of an object, and a useobserves an enlarged imaginary image of the thus formed real image. Theocular optical system 62 comprises a refractive lens 52 and adiffractive optical element 42 according to the invention. Also in thiscase, the same advantage as that of the image pick-up device shown inFIG. 22 can be attained.

Now several embodiments of the diffractive optical element according tothe invention will be explained in detail.

FIG. 8 shows a first embodiment of the diffractive optical elementaccording to the invention. The embodiment is a transmission typediffractive lens. A first optical region 101 is made of an optical glassLaL14 (trade name; nd=1.6968, υd=55.5) manufactured and sold by OHARAcompany, the second optical region 102 is made of a UV curable resin(nd=1.52, υd=52), and the third optical region 103 is made of apolycarbonate (nd=1.58, υd=30.5). These optical regions are stacked oneon the other. A first relief pattern 201 is formed in a boundary surfacebetween the first and second optical regions 101 and 102, and a secondrelief pattern 202 is formed in a boundary surface between the secondand third optical regions 102 and 103. The first and second reliefpatterns 201 and 202 have an identical pitch distribution and arealigned in a direction of an optical axis in such a manner that top andbottom portions of the first relief pattern are aligned with top andbottom portions of the second relief pattern viewed in the direction ofthe optical axis (a vertical direction in FIG. 8).

The pitch distributions of the first and second relief patterns 201 and202 are optimized to have a given lens function and a depth of recessesof a sawtooth cross sectional configuration is optimized such that thediffraction efficiency of first-order becomes maximum at a wavelength ofλ=550 nm. In the present embodiment, a depth d₁ of the first reliefpattern 201 is set to 7.90 μm and a depth d₂ of the second reliefpattern 202 is set to 13.73 μm. Then, the parameter α defined by a ratioof d₂/d₁ becomes equal to about 1.74. Outermost surfaces 301 and 302 ofthe element are formed to be flat and anti-reflecting coatings areapplied on these outermost surfaces.

FIG. 9 is a graph showing a wavelength dependency of Δn₁ expressed bythe equation (10) and a wavelength dependency of Δn₂ represented by theequation (11). The wavelength dependencies are shown for a whole visiblelight range. As can be easily seen from FIG. 9, the difference inrefractive index Δn₁ between LaL14 (first optical region 101) and the UVcurable resin (second optical region 102) is positive over a wholevisible range, because the refractive index of LaL14 is higher than thatof the UV curable resin. Moreover, an Abbe's number of LaL14 isrelatively close to that of the UV curable resin, and thus thedifference in refractive index Δn₁ is slightly decreased in accordancewith an increase in a wavelength λ. On the other hand, the difference inrefractive index Δn₂ between the UV curable resin (second optical region102) and the polycarbonate (third optical region 103) is negative over awhole visible range, because the refractive index of polycarbonate ishigher than that of the UV curable resin. Therefore, the difference inrefractive index Δn₂ becomes increased relatively largely in accordancewith an increase in a wavelength λ.

In the present embodiment, α is selected to be such a positive valuethat αΔn₂ does not exceed Δn₁ as can be understood from the graph shownin FIG. 9. Therefore, a wavelength dependency of the term of thedifference in refractive index N(λ) in the equation (15) is increased inaccordance with an increase in a wavelength λ. Then, λ in thedenominator in the equation (13) is advantageously canceled by the termN(λ), and thus a wavelength dependency of the phase amplitude isreduced. Therefore, the wavelength dependency of the diffractionefficiency is decreased.

FIG. 10 shows wavelength dependencies of the diffractive lens of thepresent embodiment and a known diffractive lens. In FIG. 10, a solidcurve denotes the wavelength dependency of the diffractive lens of thepresent embodiment, and a broken curve represents the wavelengthdependency of the known diffractive lens. In the known diffractive lens,a brazed pattern is formed on a LaL14 substrate in such a manner that afirst-order diffraction efficiency becomes maximum at a wavelength of510 nm. As can be easily understood, according to the invention, thewavelength dependency of phase amplitude is efficiently reduced.

FIG. 11 shows wavelength dependencies of diffraction efficiency of thepresent embodiment (solid curve) and the known diffractive lens (brokencurve). Also as can be understood from FIG. 11, in the diffractive lensaccording to the invention, the wavelength dependency of diffractionefficiency is corrected very well as compared with the known diffractivelens.

As explained above, in the diffractive lens of the present embodiment,it is possible to obtain a high diffraction efficiency over a wholevisible wavelength range, and therefore it is possible to avoideffectively undesired flare and ghost. The diffractive lens of thepresent embodiment may be advantageously utilized in an image pick-updevice such as camera.

As shown in FIGS. 10 and 11, in the diffractive lens of the presentembodiment, a wavelength at which the first-order diffraction efficiencybecomes 100% differs from that of the known diffractive lens. This isdue to the fact that a wavelength for optimization is selected such thatthe diffraction efficiency is balanced over a whole wavelength range tobe used. In the diffractive lens of the present embodiment, theoptimized wavelength is 550 nm, while the optimized wavelength of theknown diffractive lens is 510 nm.

In the present embodiment, the wavelength dependency of diffractionefficiency on a shorter wavelength side is reduced much more effectivelythan that on a longer wavelength side. Therefore, it is preferable toset the optimized wavelength at a longer wavelength side. In this case,it is preferable the set the optimizing wavelength to a value within arange of ±10% with respect to a center wavelength of the wavelengthrange to be used. The wavelength range for the image pick-up opticalsystem is generally set to 400-700 nm.

Furthermore, in the present embodiment, the second optical region 102 ismade of the UV curable resin, and thus it is possible to manufacture thediffractive lens in a very simple manner by forming the first and secondrelief patterns 201 and 202 in free surfaces of the first and thirdoptical regions 101 and 103, respectively and then by cementing theseoptical regions by means of the UV curable resin serving as an adhesive.In this manner, the diffractive lens of the present embodiment can bemanufactured at a low cost.

It should be noted that the above mentioned merit can be obtained mosteffectively when the second optical region 102 is made of the UV curableresin, but a similar advantage can be attained when the second opticalregion 102 is made of a plastic material.

Furthermore, in case of stacking the first and second relief patterns201 and 202, moire fringe may be utilized for aligning these patterns.That is to say, when these patterns are aligned in such a manner thatthe moire fringe does not produced, the corresponding portions of thesepatterns can be precisely aligned.

FIG. 12 shows a modification of the first embodiment. In thisdiffractive lens, one of the outermost surfaces 301 is shaped into acurve having a positive refractive power and the other outermost surface304 is curved to have a negative refractive power. The remainingstructure of this alternative embodiment is identical with theembodiment illustrated in FIG. 8. Anti-reflection coatings are appliedon the outermost surfaces 303 and 304.

The diffractive lens of the present embodiment has a power due to thediffraction and a power due to the refraction, and thus can have arelatively high power. The wavelength dispersion (Abbe's number) of thediffractive power and that of the refractive power appear in oppositesigns, so that the wavelength dispersions are canceled and a chromaticaberration can be corrected. In the embodiment illustrated in FIG. 12,the outermost surfaces 303 and 304 have refractive powers havingopposite signs, and therefore the diffractive lens can be an achromaticsingle lens in which the chromatic aberration is corrected to asecondary spectrum.

FIG. 13 depicts a second embodiment of the diffractive optical elementaccording to the invention. The diffractive optical element of thepresent embodiment is a transmission type diffractive lens. A firstoptical region 104 is made of a fluorine-contained resin “sitop” (tradename) manufactured by ASAHI GLASS company (nd=1.34149, υd=93.8)manufactured and sold by OHARA company, a second optical region 105 ismade of a UV curable resin (nd=1.52, υd=51.8), and a third opticalregion 106 is made of a polycarbonate (nd=1.58, υd=30.5). These opticalregions are stacked one on the other. A first relief pattern 203 isformed in a boundary surface between the first and second opticalregions 104 and 105, and a second relief pattern 204 is formed in aboundary surface between the second and third optical regions 105 and106. The first and second relief patterns 203 and 204 have an identicalpitch distribution and are aligned in a direction of an optical axis insuch a manner that top and bottom portions of the first relief patternare aligned with bottom and top portions of the second relief patternviewed in the direction of the optical axis.

The pitch distribution of the first and second relief patterns 203 and204 is optimized to have a given lens function and a depth of recessesof a sawtooth cross sectional configuration is optimized such that thediffraction efficiency of first-order becomes maximum at a wavelength ofλ=550 nm. In the present embodiment, a depth d₁ of the first reliefpattern 203 and a depth d₂ of the second relief pattern 204 haveopposite signs, because the these patterns are arranged in a relation ofup-side-down. The depth of the first relief patterns 203 is set to −9.2μm and the depth of the second relief pattern 204 is set to 17.84 μm.Then, the parameter α defined by a ratio of d₂/d₁ becomes equal to about−1.94. Outermost surfaces 305 and 306 of the element are formed to beflat and anti-reflecting coatings are applied on these outermostsurfaces.

FIG. 14 is a graph showing a wavelength dependency of Δn₁ expressed bythe equation (10) and a wavelength dependency of Δn₂ represented by theequation (11) in the present embodiment. As can be seen from FIG. 14,the difference in refractive index Δn₁ between “sitop” (first opticalregion 104) and the UV curable resin (second optical region 105) isnegative for a visible wavelength range, and similarly the difference inrefractive index Δn₂ between the UV curable resin (second optical region105) and the polycarbonate (third optical region 106) is negative for avisible wavelength range. Moreover, this structure is a combination ofhigh refractive index and high dispersion and a low refractive index andlow dispersion. Therefore, the differences in refractive index Δn₁ andΔn₂ (absolute values) are decreased in accordance with an increase in awavelength λ.

In the present embodiment, α is selected to be such a negative valuethat an absolute value of αΔn₂ does not exceed an absolute value of Δn₁,and therefore, a wavelength dependency of the term of the difference inrefractive index N(λ) in the equation (15) is increased in accordancewith an increase in a wavelength λ. Then, λ in the denominator in theequation (13) is advantageously canceled by the term N(λ), and thus awavelength dependency of the phase amplitude is reduced and further thewavelength dependency of the diffraction efficiency is decreased.

FIG. 15 shows wavelength dependencies of diffraction efficiency of thediffractive lens of the present embodiment and a known diffractive lens.In FIG. 15, a solid curve denotes the wavelength dependency ofdiffraction efficiency of the diffractive lens of the presentembodiment, and a broken curve represents the wavelength dependency ofdiffraction efficiency of the known diffractive lens. In the knowndiffractive lens, a brazed pattern is formed on a “sitop” substrate(optimizing wavelength is 510 nm). As can be seen from FIG. 15, in thediffractive lens according to the invention, the wavelength dependencyof diffraction efficiency is effectively reduced as compared with theknown diffractive lens.

As explained above, in the diffractive lens of the present embodiment,it is possible to obtain a high diffraction efficiency over a wholevisible wavelength range, and therefore a problem of flare and ghost canbe solved. The diffractive lens of the present embodiment may beadvantageously utilized in an image pick-up device-such as camera.

In the present embodiment, the first and third optical regions 104 and106 are made of plastic materials, and thus it is possible tomanufacture the first and second relief patterns 203 and 204 in a verysimple manner. surfaces of the first and third optical regions 101 and103, respectively and then by cementing these optical regions by meansof the UV curable resin serving as an adhesive. In this manner, thediffractive lens of the present embodiment can be manufactured at a lowcost.

Further, since the second optical region 105 is made of the UV curableresin, the diffractive lens can be manufactured very easily by formingthe first and second relief patterns 203 and 204 on the first and secondoptical regions 104 and 105, respectively and then by cementing theseoptical regions 104 and 105 by means of the UV curable resin. In thismanner, the diffractive lens having the reduce wavelength dependency ofdiffraction efficiency can be manufactured easily at a low cost.

FIG. 16 shows a third embodiment of the diffractive optical elementaccording to the invention. the embodiment is a double-focus typediffractive lens having. The layered structure of the present embodimentis same as that of the first embodiment. That is to say, a first opticalregion 101 is made of an optical glass LaL14 (nd=1.6968, υd=55.5)manufactured and sold by OHARA company, the second optical region 102 ismade of a UV curable resin (nd=1.52, υd=52), and the third opticalregion 103 is made of a polycarbonate (nd=1.58, υd=30.5). A first reliefpattern 205 is formed in a boundary surface between the first and secondoptical regions 101 and 102, and a second relief pattern 206 is formedin a boundary surface between the second and third optical regions 102and 103. The first and second relief patterns 205 and 206 have anidentical pitch distribution and are aligned in a direction of anoptical axis in such a manner that top and bottom portions of the firstrelief pattern are aligned with bottom and top portions of the secondrelief pattern in the direction of the optical axis.

The pitch distribution of the first and second relief patterns 205 and206 is optimized to have a given lens function. These relief patterns205 and 206 are formed into a cross sectional shape having rectangulardepressions and protrusions, a ratio of the depressions to theprotrusions is equal to unity. A depth of the depressions andprotrusions is optimized such that the diffraction efficiency offirst-order becomes maximum at a wavelength of λ=600 nm. In the presentembodiment, a depth d₁ of the first relief pattern 205 is set to 4.02 μmand a depth d₂ of the second relief pattern 206 is set to 7.03 μm. Then,the parameter α defined by a ratio of d₂/d₁ becomes equal to about 1.75.Outermost surfaces 301 and 302 of the element are formed to be flat andanti-reflecting coatings are applied on these outermost surfaces.

In the present embodiment, when ±1-order diffraction efficiencies becomemaximum, the phase amplitude corresponding to the equation (13) may beexpressed as follows:

$\begin{matrix}{{a(\lambda)} = {\frac{m_{0}}{2} \cdot \frac{\left\{ {{n_{1}(\lambda)} - {n_{2}(\lambda)}} \right\} + {\alpha\left\{ {{n_{2}(\lambda)} - {n_{3}(\lambda)}} \right\}}}{\left\{ {{n_{1}\left( \lambda_{0} \right)} - {n_{2}\left( \lambda_{0} \right)}} \right\} + {\alpha\left\{ {{n_{2}\left( \lambda_{0} \right)} - {n_{3}\left( \lambda_{0} \right)}} \right\}}} \cdot \frac{\lambda_{0}}{\lambda}}} & (21)\end{matrix}$

Then, the diffraction efficiency for m-order ηm may be represented bythe following equation (22):

$\begin{matrix}{\eta_{m} = \left\{ {{\frac{\sin\left( {m\;{\pi/2}} \right)}{m\;{\pi/2}} \cdot \cos}\;{\pi\left( {a + \frac{m}{2}} \right)}} \right\}^{2}} & (22)\end{matrix}$

The phase amplitude denoted by the equation (21) may be obtained bydiving the right hand term in the equation (13) by two. This means thatalso in the present embodiment, the wavelength dependency of phaseamplitude can be reduced by the same function as that of the firstembodiment.

FIG. 17 shows the wavelength dependency of ±1-order diffractionefficiency of the diffractive lens of the present embodiment (solidcurve) and a wavelength dependency of a known diffractive lens in whicha rectangular phase grating is formed in a LAL14 substrate (brokencurve). From the graph shown in FIG. 17, it is apparent that accordingto the invention, the wavelength dependency of diffraction efficiency iscorrected very well. The wavelength dependency of diffraction efficiencyis corrected over a whole visible wavelength range, and thus thediffractive lens of the present embodiment is preferably used as adouble-focus optical system for a visible wavelength range.

In the above mentioned first and second embodiments, the first andsecond relief patterns are formed to have the sawtooth cross sectionalconfiguration, and in the third embodiment, the first and second reliefpatterns are shaped to have the rectangular cross sectionalconfiguration, However, according to the invention, the cross sectionalconfiguration of the first and second relief patterns is not restrictedto these configurations, but any other configuration may be applied.

FIG. 18 illustrates a fourth embodiment of the diffractive opticalelement according to the invention. the diffractive optical element ofthe present embodiment is formed as a transmission type diffractivelens. A first optical region 107 is made of an acrylic resin (nd=1.49,υd=57.7), and a second optical region 108 is made of a polycarbonate(nd=1.58, υd=30.5). These optical regions are stacked one on the other.In the present embodiment, the third optical region is constituted bythe air surrounding the first and second optical regions 107 and 108. Afirst relief pattern 207 is formed in a boundary surface between thefirst and second optical regions 107 and 108, and a second reliefpattern 208 is formed in a boundary surface between the second opticalregion 108 and the air, i.e. the third optical region. That is to say,the second relief pattern 208 is formed in a free surface of the secondoptical region 108. The first and second relief patterns 207 and 208have an identical pitch distribution and are aligned in a direction ofan optical axis in such a manner that top and bottom portions of thefirst relief pattern are aligned with bottom and top portions of thesecond relief pattern viewed in the direction of the optical axis. Inthe present embodiment, the top portions of the first relief pattern 207are brought into contact with the bottom portions of the second reliefpattern 208.

The pitch distribution of the first and second relief patterns 207 and208 is optimized to have a given light collimating function and a depthof recesses of a sawtooth cross sectional configuration is optimizedsuch that the diffraction efficiency of first-order becomes maximum at awavelength of λ=550 nm. In the present embodiment, a depth d₁ of thefirst relief pattern 207 is 15.6 μm and a depth d₂ of the second reliefpattern 208 is 3.34 μm. Therefore, the parameter α defined by a ratio ofd₂/d₁ is equal to about 0.22.

In the present embodiment, the third optical region is formed by theatmosphere surrounding the optical element, practically the air having arefractive index of 1. Therefore, the wavelength dependency ofdiffraction efficiency can be corrected by the same function as theprevious embodiments. Particularly, the third optical region has a verylow refractive index, and thus a difference in refractive index betweenthe second optical region 108 and the third optical region Δn₂ issufficiently large. Then, a depth of the second relief pattern 208 canbe relatively small. Therefore, the diffractive lens of the presentinvention can be thin, and the pitch of the relief patterns can besmall.

In FIG. 19, a solid curve denotes the wavelength dependency ofdiffraction efficiency of the diffractive lens of the present embodimentand a broken curve shows a wavelength dependency of diffractionefficiency of a known diffractive lens in which a brazed pattern isformed in an acrylic resin substrate (optimizing wavelength λ=510 nm).From the graph shown in FIG. 19, it is apparent that also in the presentembodiment, the wavelength dependency of diffraction efficiency iscorrected very well.

FIG. 20 shows a fifth embodiment of the diffractive optical elementaccording to the second aspect of the invention. The diffractive opticalelement of the present embodiment is formed as a reflection typediffractive optical element. A first optical region 111 is made of analuminum, a second optical region 112 is made of a polycarbonate(nd=1.58, υd=30.5), and a third optical region 113 is made of an acrylicresin (nd=1.49, υd=57.5). These optical regions are stacked one on theother. A first relief pattern 211 is formed in a boundary surfacebetween the first and second optical regions 111 and 112, and a secondrelief pattern 212 is formed in a boundary surface between the secondoptical region 112 and the third optical region 113. The first andsecond relief patterns 211 and 212 have an identical pitch distributionand are aligned in a direction of an optical axis in such a manner thattop and bottom portions of the first relief pattern are aligned withbottom and top portions of the second relief pattern viewed in thedirection of the optical axis. In the present embodiment, the first andsecond relief patterns 211 and 212 are formed to overlap with eachother, and thus a distance d₃ between these relief patterns has anegative value.

The first and second relief patterns 211 and 212 are formed into asawtooth cross sectional configuration having identical pitches and adepth of recesses of the sawtooth cross sectional configuration isoptimized such that the diffraction efficiency of first-order becomesmaximum at a wavelength of λ=550 nm. In the present embodiment, a depthd₁ of the first relief pattern 211 is 0.53 μm and a depth d₂ of thesecond relief pattern 212 is 6.04 μm. Therefore, the parameter α definedby a ratio of d₂/d₁ is equal to about 11.40.

In the present embodiment, the first optical region 111 is made of areflecting material, i.e. aluminum, and therefore light being madeincident from an incident surface 311 is reflected by the first reliefpattern 212. Therefore, the diffractive optical element of the presentembodiment operates as a reflection type diffractive optical element.The wavelength dependency of diffraction efficiency can be explained bythe equation (14). This equation (14) may be derived by setting zero therefractive index of the first optical region in the equation (13) tozero, said equation (13) representing the phase amplitude of thetransmission type diffractive optical element. Therefore, the wavelengthdependency of diffraction efficiency can be corrected by the samefunction as the previous embodiments. Particularly, the third opticalregion has a very low refractive index, and thus a difference inrefractive index between the second optical region 108 and the thirdoptical region Δn₂ is sufficiently large. Then, a depth of the secondrelief pattern 208 can be relatively small. Therefore, the diffractivelens of the present invention can be thin, and the pitch of the reliefpatterns can be small.

In FIG. 21, a solid curve denotes the wavelength dependency ofdiffraction efficiency of the reflection type diffractive opticalelement of the present embodiment and a broken curve shows a wavelengthdependency of diffraction efficiency of a known reflection typediffractive optical element in which a reflection type brazed grating isformed (optimizing wavelength λ=510 nm). From FIG. 21, it can beunderstood that in the present embodiment, the wavelength dependency ofdiffraction efficiency is corrected very well as compared with the knowndiffractive optical element.

FIG. 24 is a schematic cross sectional view illustrating a principalstructure of the diffractive optical element according to the fourthaspect of the invention. The diffractive optical element comprisesfirst, second, third and fourth optical regions 121, 122, 123 and 124,which are stacked one on the other successively, a first relief pattern221 formed in a boundary surface between the first and second regions121 and 122, and a second relief pattern 222 is formed in a boundarysurface between the third and fourth optical regions 123 and 124. Thefirst, second, third and fourth optical regions 121-124 are made of atleast three different kinds of optical materials which are substantiallytransparent to light within a wavelength range to be used. The first andsecond optical regions 121 and 122 are made of different opticalmaterials, and the third and fourth optical regions 123 and 124 are madeof different optical materials. Refractive indices of these opticalregions 121-124 are represented by n₁, n₂, n₃ and n₂, respectively.

The first and second relief patterns 221 and 222 are formed to have asawtooth cross sectional configuration having the same pitchdistribution. A depth of the first sawtooth relief pattern 221 is d₁,and a depth of the second sawtooth relief pattern 222 is d₂, a distancebetween a top of the first relief pattern 221 and a boundary surfacebetween the second and third optical regions 122 and 123 is d₃, and adistance between this boundary surface and bottom portions of the secondrelief pattern 222 is d₄.

In the diffractive optical element shown in FIG. 24, light is madeincident upon the diffractive optical element along an optical axisthereof which is a vertical direction in a plane of the drawing of FIG.24. Then, the incident light is subjected to a phase modulation by thefirst and second relief patterns 221 and 222. In this case, a phaseamplitude a₁(λ)′ of the first relief pattern 221 may be expressed asfollows:

$\begin{matrix}{{a_{1}(\lambda)} = {{\frac{\Delta\;{n_{1}(\lambda)}}{\lambda}d_{1}\text{;}\mspace{14mu}\Delta\;{n_{1}(\lambda)}} = {{n_{1}(\lambda)} - {n_{2}(\lambda)}}}} & (23)\end{matrix}$

A phase amplitude a₂(λ) of the second relief pattern 222 may berepresented in the following manner:

$\begin{matrix}{{a_{2}(\lambda)} = {{\frac{\Delta\;{n_{2}(\lambda)}}{\lambda}d_{2}\text{;}\mspace{14mu}\Delta\;{n_{2}(\lambda)}} = {{n_{3}(\lambda)} - {n_{4}(\lambda)}}}} & (24)\end{matrix}$

Now it is assumed that the first and second relief patterns 221 and 222are constructed into a single integral body and the incident light ismodulated substantially at the same time. Then, a total phase amplitudea(λ) which characterizes the phase shift function may be expressed bythe following equation:

$\begin{matrix}{{a(\lambda)} = {{{a_{1}(\lambda)} + {a_{2}(\lambda)}} = {\frac{{\Delta\;{n_{1}(\lambda)}} + {{\alpha\Delta}\;{n_{2}(\lambda)}}}{\lambda}d_{1}}}} & (25)\end{matrix}$

When a depth of the sawtooth relief patterns 221 and 222 is optimizedsuch that a diffraction efficiency of m₀-order becomes 100% for aspecific wavelength λ₀, a(λ₀)=m₀ is obtained. Then, the followingequation (26) may be obtained:

$\begin{matrix}{{a(\lambda)} = {m_{0} \cdot \frac{{\Delta\; n_{1}(\lambda)} + {{\alpha\Delta}\;{n_{2}(\lambda)}}}{{\Delta\;{n_{1}\left( \lambda_{0} \right)}} + {{\alpha\Delta}\;{n_{2}\left( \lambda_{0} \right)}}} \cdot \frac{\lambda_{0}}{\lambda}}} & (26)\end{matrix}$wherein α is a value defined by a ratio of the depth d₂ of the secondrelief pattern 222 to the depth d₁ of the first relief pattern 221 asfollows:

$\begin{matrix}{\alpha = \frac{d_{2}}{d_{1}}} & (27)\end{matrix}$

As expressed by the equation (25), the phase amplitude a(λ) of thediffractive optical element according to the fourth aspect of theinvention is given by a sum of the phase amplitude a₁(λ) of the firstrelief pattern 221 and the phase amplitude a₂(λ) of the second reliefpattern 222, and a wavelength dependency of the phase amplitude a(λ) ofthe diffractive optical element is dependent upon the parameter αdefined by the equation (27). Here, as can be seen from the equation(26), the parameter α′ may be determined at will in regardless with theoptimization of the diffraction efficiency for the specific wavelengthλ₀.

In the diffractive optical element illustrated in FIG. 24, the first,second, third and fourth optical regions 121, 122, 123 and 124 are madeof different optical materials which are substantially transparent tolight within a given wavelength to be used, and the differences inrefractive index Δn₁′ and Δn₂ have different wavelength dependenciesfrom each other. Therefore, by changing the parameter α, the wavelengthdependency of the phase amplitude a(λ) defined by the equation (26) canbe adjusted in various ways.

According to the fourth aspect of the invention, only the wavelengthdependency of the diffraction efficiency at the specific wavelength λ₀can be exclusively adjusted or controlled in a favorable manner bysuitably setting the ratio of the depth of the second relief pattern tothe depth of the first relief pattern, i.e. the parameter α, while adiffractive efficiency at the specific wavelength λ₀ can be maintainedto be optimum. In general, it is possible to optimize the wavelengthdependency of diffraction efficiency by making the depths different fromeach other (α≠1). However, according to the invention, it is alsopossible to optimize the wavelength dependency of diffraction efficiencyby suitably combining the optical materials of the first, second, thirdand fourth optical regions even under a condition that α=1.

FIG. 25 is a schematic cross sectional view illustrating the diffractiveoptical element according to the fifth aspect of the invention. Thediffractive optical element comprises first, second, third and fourthoptical regions 125, 126, 127 and 128 which are stacked one on the othersuccessively, a first relief pattern 223 formed in a boundary surfacebetween the first and second optical regions 125 and 126, and a secondrelief pattern 224 formed in a boundary surface between the third andfourth optical regions 127 and 128. The first optical region 125 is madeof a material which reflects light within a wavelength range to be used.The second, third and fourth optical regions 126, 127 and 128 are madeof at least three different kinds of optical materials which aresubstantially transparent to light within the wavelength range to beused. The third and fourth optical regions 127 and 128 are made ofdifferent optical materials.

The second, third and fourth optical regions 126, 127 and 128 haverefractive indices n₂, n₃ and n₄, respectively.

The first and second relief patterns 223 and 224 are formed to have asawtooth cross sectional configuration having the same pitchdistribution. A depth of the first sawtooth relief pattern 223 is d₁ anda depth of the second sawtooth relief pattern 224 is d₂. Further, adistance from top portions of the first relief pattern 223 to a boundarysurface between the second and third optical regions 126 and 127 is d₃,and a distance from said boundary surface to bottom portions of thesecond relief pattern 224 is d₄.

In FIG. 25, light is made incident upon the diffractive optical elementfrom the side of the fourth optical region 128 and is subjected to thephase modulation by the second and first relief patterns 224 and 223 inthis order. The first relief pattern 223 is formed on the surface of thefirst optical region 125 made of the material which reflects the light,and thus the light is reflected by the first relief pattern 223. In thismanner, the diffractive optical element of this embodiment serves as areflection type diffractive optical element.

In the present diffractive optical element, it is also assumed that thefirst and second relief patterns 223 and 224 are constructed as a singleintegral body, so that the light impinging upon the diffractive opticalelement is modulated substantially simultaneously. Then, a phaseamplitude of the diffractive optical element may be expressed by thefollowing equation (28) when the depths of the first and second reliefpatterns 223 and 224 are set in such a manner that the diffractionefficiency of m₀ for a specific wavelength λ₀ becomes 100%.

$\begin{matrix}{{a(\lambda)} = {m_{0} \cdot \frac{{{- n_{2}}(\lambda)} + {\alpha\left\{ {{n_{3}(\lambda)} - {n_{4}(\lambda)}} \right\}}}{{- {n_{2}\left( \lambda_{0} \right)}} + {\alpha\left\{ {{n_{3}\left( \lambda_{0} \right)} - {n_{4}\left( \lambda_{0} \right)}} \right\}}} \cdot \frac{\lambda_{0}}{\lambda}}} & (28)\end{matrix}$

This equation (28) corresponds to the equation (26) of the structureshown in FIG. 24 first embodiment and the parameter α is a ratio of thedepth d₂ of the second relief pattern 224 to the depth d₁ of the firstrelief pattern 223 as defined by the equation (27).

The above mentioned equation (28) may be obtained by setting therefractive index of the first optical region 14 in the equation (26) ofthe structure shown in FIG. 24 to zero. That is to say, also in thepresent structure, the phase amplitude can be expressed by the parameterα which may take an arbitrary value. Therefore, also in the presentdiffractive optical element, by suitably setting the parameter α, it ispossible to control only the wavelength dependency of diffractionefficiency, while keeping the diffraction efficiency for the specificwavelength optimum like as the diffractive optical element shown in FIG.24.

In the fourth aspect of the invention, in order to further reduce thewavelength dependency of diffraction efficiency, it is necessary todecrease the wavelength dependency of phase amplitude a(λ) defined bythe equation (25) or (26). For instance, in the equation (26), thewavelength dependency of phase amplitude a(λ) is determined by the twoterms including the differences in refractive index Δn₁(λ) and Δn₂(λ) inthe numerator and λ in the denominator. Therefore, in order to reducethis wavelength dependency, it is preferable to select the opticalmaterials and said ratio of the depths in such a manner that an absolutevalue of a sum of the two differences in refractive index ΔN(λ) isincreased in accordance with an increase in the wavelength λ. Saidabsolute value of a sum of the two differences in refractive index ΔN(λ)can be expressed by the following equation (29):ΔN(λ)=|Δn ₁(λ)+αΔn ₂(λ)|  (29)

Then, the numerator and denominator in the equation (26) have oppositefunctions for the wavelength dependency of diffraction efficiency, andthus it is possible to realize the diffractive optical element having afurther reduced wavelength dependency of diffraction efficiency. Theequation (26) is defined for the structure according to the fourthaspect of the invention, but the equation (28) may be obtained bysetting n1(λ)=0 in the equation (26), so that the explanation about theequation (29) may be also applied to the fifth aspect of the presentinvention.

In case of actually selecting optical materials for the diffractiveoptical element, there are many optical materials, in which an absolutevalue of the difference in refractive index Δn(λ) becomes decreased inaccordance with an increase in the wavelength λ. This does not result inthe desired wavelength dependency. That is to say, many availableoptical materials are distributed from a range of high refractive indexand high dispersion to a range of low refractive index and lowdispersion. In the fourth and fifth aspects of the invention, it iseffective to setting a sign, i.e. positive or negative of the ratio ofdepths of the grooves of the relief patterns such that the two termsincluding the difference in refractive index are canceled each other.Then, the wavelength dependency opposite to the desired property can becanceled out. In this manner, by combining optical materials having highrefractive index and high dispersion with optical materials having lowrefractive index and low dispersion, the wavelength dependency ofdiffraction efficiency can be further reduced, and such a combinationcan be easily realized.

The sign of the ratio α can be determined by a manner of arranging thefirst and second relief patterns. When the first and second reliefpatterns are arranged such that tops and bottoms of the first patternare aligned with tops and bottoms of the second pattern in the directionof the optical axis as illustrated in FIG. 26, the ratio α is positive.When the first and second relief patterns are arranged such that topsand bottoms of the first pattern are aligned with bottoms and tops ofthe second pattern as shown in FIG. 31, it is possible to obtain anegative ratio α.

In The forth to sixth aspects of the invention, no relief pattern isformed in the boundary surface between the second and third opticalregions, and thus these optical regions may be formed by the sameoptical material. This is very advantageous in view of manufacture.

In the fourth to sixth aspects of the invention, the fourth opticalregion may be formed by an atmosphere surrounding the diffractiveoptical element. In general, the diffractive optical element is placedin the air, the fourth optical region may be made of the air. In thiscase, a difference in refractive index between the third and fourthoptical regions can be very large, and thus a necessary depth of therelief pattern may be small and a diffractive optical element having asuperior property can be realized.

In general, the diffractive optical element may be classified into athick type and a thin type. In case realizing a diffractive opticalelement for an image focusing optical system using light rays within awavelength range, it is preferable to use the thin type one which has arelatively low incident angle dependency as well as a relatively lowwavelength dependency. It has been well known to use a parameter Qcharacterizing a thickness of the diffractive optical element, saidparameter being defined as follows:

$\begin{matrix}{Q = \frac{2\pi\;\lambda\; D}{n_{0}T^{2}}} & (30)\end{matrix}$

In general, when Q<1, a diffractive optical element is classified intothe thin type. In the above equation (30), T is a pitch of a periodicstructure of a relief pattern and n₀ is an average refractive index ofthe periodic structure. In the present invention, it is also preferableto construct the diffractive optical element to satisfy the conditionQ<1.

From the equation (30), it is understood that the parameter Q denotingthe thickness of the diffractive optical element is dependent upon thewavelength λ. However, in order to keep a uniformity of the diffractionefficiency over a whole wavelength range to be used, it is sufficientthat the above condition Q<1 is satisfied for a center wavelength withinthe whole wavelength range. Therefore, also in the present invention, itis preferable to construct the diffractive optical element such that thecondition Q<1 is satisfied for a center wavelength. For instance, whenthe diffractive optical element is used for a visible light wavelength,the center wavelength may be set to a value within a range from 480 nmto 550 nm. Of course, it is much more preferable to construct thediffractive optical element to satisfy the condition Q<1 for the wholewavelength range.

The inventor has confirmed that when Q<0.1, the relief type diffractiveoptical element can have characteristics of the thin type in anefficient manner. Therefore, according to the invention, it ispreferable to construct the periodic structure to satisfy the conditionQ<0.1.

In the embodiment shown in FIG. 24, the depth D and average refractiveindex n₀ of the periodic structure may be expressed as follows:D=d ₁ +d ₂ +d ₃ +d ₄  (31)

$\begin{matrix}{n_{0} = \frac{{d_{1}\frac{n_{1} + n_{2}}{2}} + {d_{2}\frac{n_{3} + n_{4}}{2}} + {d_{3}n_{2}} + {d_{4}n_{3}}}{d_{1} + d_{2} + d_{3} + d_{4}}} & (32)\end{matrix}$

In the embodiment shown in FIG. 25, the depth D and average refractiveindex n₀ of the periodic structure may be expressed as follows:D=2(d ₁ +d ₂ +d ₃ +d ₄)  (33)

$\begin{matrix}{n_{0} = \frac{{d_{1}\frac{n_{2}}{2}} + {d_{2}\frac{n_{3} + n_{4}}{2}} + {d_{3}n_{2}} + {d_{4}n_{3}}}{d_{1} + d_{2} + d_{3} + d_{4}}} & (34)\end{matrix}$

It should be noted that the diffractive optical element according to theinvention is particularly suitable for a wavelength range wider than agiven amount. In conventional diffractive optical element in which thediffraction efficiency is optimized for an arbitrary wavelength λ, awidth of a wavelength range in which a variation of the diffractionefficiency can be neglected is about 5%. Therefore, the diffractiveoptical element according to the invention can be advantageously usedfor a wavelength range which is wider than 5% of a center wavelength λ.

The above explanation is described for a case in which the first, secondand third optical regions are brought into contact with each other.However, this may be also applied to a diffractive optical element inwhich said optical regions are brought closer to each other by means ofcementing layers provided between adjacent optical regions.

The diffractive optical element according to the invention may be usedfor general optical systems in which a plurality of wavelengths within awavelength range are used, but the diffractive optical element accordingto the invention can be particularly advantageously used in a visiblelight image focusing optical system. That is to say, the diffractiveoptical elements according to the fourth to fifth aspects of theinvention can be advantageously used also for the optical systems shownin FIGS. 22 and 23.

Now several embodiments of the diffractive optical element according tothe fourth to sixth aspects of the invention will be explained indetail.

FIG. 26 illustrates a fifth embodiment of the diffractive opticalelement according to fourth aspect of the invention. The embodiment is atransmission type diffractive lens. A first optical region 131 is madeof an optical glass LaL14 (nd=1.6968, υd=55.5) manufactured and sold byOHARA company, second and third optical regions 132 and 133 are made ofa UV curable resin (nd=1.52, υd=52), and a fourth optical region 134 ismade of a polycarbonate (nd=1.58, υd=30.5). These optical regions131-134 are stacked one on the other. A first relief pattern 231 isformed in a boundary surface between the first and second opticalregions 131 and 132, and a second relief pattern 232 is formed in aboundary surface between the third and fourth optical regions 133 and134. The first and second relief patterns 231 and 232 have an identicalpitch distribution and are aligned in a direction of an optical axis insuch a manner that top and bottom portions of the first relief patternare aligned with top and bottom portions of the second relief patternviewed in the direction of the optical axis.

The pitch distributions of the first and second relief patterns 231 and232 are optimized to have a given lens function and a depth of recessesof a sawtooth cross sectional configuration is optimized such that thediffraction efficiency of first-order becomes maximum at a wavelength ofλ=550 nm. In the present embodiment, a depth d₁ of the first reliefpattern 231 is set to 7.90 μm and a depth d₂ of the second reliefpattern 232 is set to 13.74 μm. Then, the parameter α defined by a ratioof d₂/d₁ as shown by the equation (27) becomes equal to about 1.74.Outermost surfaces 321 and 322 of the diffractive optical element areformed to be flat and anti-reflecting coatings are applied on theseoutermost surfaces.

FIG. 27 is a graph showing a wavelength dependency of Δn₁ expressed bythe equation (23) and a wavelength dependency of Δn₂ represented by theequation (24). The wavelength dependencies are shown for a whole visiblelight range. As can be understood from FIG. 27, the difference inrefractive index Δn₁ between LaL14 (first optical region 131) and the UVcurable resin (second optical region 132) is positive over a wholevisible range, because the refractive index of LaL14 is higher than thatof the UV curable resin. Moreover, an Abbe's number of LaL14 isrelatively close to that of the UV curable resin, and thus thedifference in refractive index Δn₁ is slightly decreased in accordancewith an increase in a wavelength λ. On the other hand, the difference inrefractive index Δn₂ between the UV curable resin (third optical region133) and the polycarbonate (fourth optical region 134) is negative overa whole visible range, because the refractive index of polycarbonate ishigher than that of the UV curable resin. Therefore, the difference inrefractive index Δn₂ increases relatively largely in accordance with anincrease in a wavelength λ.

In the present embodiment, α is selected to be such a positive valuethat αΔn₂ does not exceed Δn₁ as can be understood from the graph shownin FIG. 27. Therefore, a wavelength dependency of the term of thedifference in refractive index N(λ) in the equation (29) is increased inaccordance with an increase in a wavelength λ. Then, λ in thedenominator in the equation (26) is advantageously canceled by the termN(λ), and thus a wavelength dependency of phase amplitude is reduced andfurther the wavelength dependency of diffraction efficiency isdecreased.

FIG. 28 shows wavelength dependencies of diffractive lens of the presentembodiment and a known diffractive lens. In FIG. 28, a solid curveexpresses the wavelength dependency of diffraction efficiency of thediffractive lens of the present embodiment, and a broken curverepresents the wavelength dependency of diffraction efficiency of theknown diffractive lens. In the known diffractive lens, a brazed patternis formed on a LaL14 substrate in such a manner that a first-orderdiffraction efficiency becomes maximum at a wavelength of 510 nm. As canbe easily understood, according to the invention, the wavelengthdependency of phase amplitude is efficiently reduced by optimizing thevalue of α.

FIG. 29 shows wavelength dependencies of diffraction efficiency of thepresent embodiment (solid curve) and the known diffractive lens (brokencurve). Also as can be understood from FIG. 29, in the diffractive lensaccording to the invention, the wavelength dependency of diffractionefficiency is corrected very well as compared with the known diffractivelens.

As explained above, in the diffractive lens of the present embodiment,it is possible to obtain a high diffraction efficiency over a wholevisible wavelength range, and therefore it is possible to avoideffectively undesired flare and ghost. The diffractive lens of thepresent embodiment may be advantageously utilized in an image pick-updevice such as camera.

As shown in FIGS. 29 and 30, in the diffractive lens of the presentembodiment a wavelength at which the diffraction efficiency offirst-order becomes 100% differs from that of the known diffractivelens. This is due to the fact that an optimizing wavelength is selectedsuch that the diffraction efficiency is well balanced over a wholewavelength range to be used. In the diffractive lens of the presentembodiment, the optimizing wavelength is 550 nm, while the optimizingwavelength of the known diffractive lens is 510 nm.

In the present embodiment, the wavelength dependency of diffractionefficiency on a shorter wavelength side is reduced much more effectivelythan that on a longer wavelength side. Therefore, it is preferable toset the optimizing wavelength at a longer wavelength side. In this case,it is preferable to set the optimizing wavelength to a value within arange of ±10% with respect to a center wavelength of the wavelengthrange to be used. The wavelength range for the image pick-up opticalsystem is generally set to 400-700 nm.

Furthermore, in the present embodiment, the second and third opticalregions 132 and 133 are made of the UV curable resin, and thus it ispossible to manufacture the diffractive lens in a very simple manner byforming the first and second relief patterns 231 and 232 in freesurfaces of the first and fourth optical regions 131 and 134,respectively and then by cementing these optical regions by means of theUV curable resin constituting the second and third optical regions 132and 133. In this manner, the diffractive lens of the present embodimentcan be manufactured at a low cost.

The above mentioned merit can be obtained most effectively when thesecond and third optical regions 132 and 233 are made of the UV curableresin, but a similar advantage can be attained when the second and thirdoptical regions 132 and 133 are made of a plastic material.

Furthermore, in case of stacking the first and second relief patterns201 and 202, moire fringe may be utilized for aligning these patterns.That is to say, when these patterns are aligned in such a manner thatthe moire fringe is completely disappeared, the corresponding portionsof these patterns can be precisely aligned.

FIG. 30 depicts modification of the six embodiment shown in FIG. 26. Inthis diffractive lens, one of the outermost surfaces 323 is shaped intoa curve having a positive refractive power and the other outermostsurface 324 is curved to have a negative refractive power. The remainingstructure of this modified embodiment is identical with the embodimentillustrated in FIG. 26. Anti-reflection coatings are applied on theoutermost surfaces 323 and 324.

The diffractive lens of the present embodiment has a power due to thediffraction and a power due to the refraction, and thus can have arelatively large power. The wavelength dispersion (Abbe's number) of thediffractive power and that of the refractive power appear in oppositesigns, so that the wavelength dispersions are canceled and a chromaticaberration can be corrected. In the embodiment illustrated in FIG. 12,the outermost surfaces 323 and 324 have refractive powers havingopposite signs, and therefore the diffractive lens can be an achromaticsingle lens in which the chromatic aberration is corrected to asecondary spectrum.

FIG. 31 shows a seventh embodiment of the diffractive optical elementaccording to the fourth aspect of the invention. The diffractive opticalelement of the present embodiment is a transmission type diffractivelens. A first optical region 135 is made of a fluorine-contained resin“sitop” manufactured by ASAHI GLASS company (nd=1.34149, υd=93.8),second and third optical regions 136 and 137 are made of a UV curableresin (nd=1.52, υd=51.8), and a fourth optical region 138 is made of apolycarbonate (nd=1.58, υd=30.5). These optical regions are stacked oneon the other. A first relief pattern 233 is formed in a boundary surfacebetween the first and second optical regions 135 and 136, and a secondrelief pattern 234 is formed in a boundary surface between the third andfourth optical regions 137 and 138. The first and second relief patterns233 and 234 have an identical pitch distribution and are aligned in adirection of an optical axis in such a manner that top and bottomportions of the first relief pattern 233 are aligned with bottom and topportions of the second relief pattern 234 viewed in the direction of theoptical axis. That is to say, the first and second relief patterns 233and 234 are arranged to be opposed to each other.

The pitch distribution of the first and second relief patterns 233 and234 is optimized to have a given lens function and a depth of recessesof a sawtooth cross sectional configuration is optimized such that thediffraction efficiency of first-order becomes maximum at a wavelength ofλ=550 nm. In the present embodiment, a depth d₁ of the first reliefpattern 203 and a depth d₂ of the second relief pattern 204 haveopposite signs, because the these patterns are arranged in a relation ofup-side-down. The depth of the first relief patterns 233 is set to −9.20μm and the depth of the second relief pattern 234 is set to 17.84 μm.Then, the parameter α defined by a ratio of d₂/d₁ becomes equal to about−1.94. Outermost surfaces 324 and 325 of the element are formed to beflat and anti-reflecting coatings are applied on these outermostsurfaces.

FIG. 32 is a graph showing a wavelength dependency of Δn₁ expressed bythe equation (23) and a wavelength dependency of Δn₂ represented by theequation (24) in the present embodiment. As can be seen from FIG. 32,the difference in refractive index Δn₁ between “sitop” (first opticalregion 135) and the UV curable resin (second optical region 136) isnegative for a visible wavelength range, and similarly the difference inrefractive index Δn₂ between the UV curable resin (third optical region137) and the polycarbonate (fourth optical region 138) is negative for avisible wavelength range. Moreover, this structure is a combination ofhigh refractive index and high dispersion and a low refractive index andlow dispersion. Therefore, the differences in refractive index Δn₁ andΔn₂ (absolute values) are decreased in accordance with an increase in awavelength λ.

In the present embodiment, α is selected to be such a negative valuethat an absolute value of αΔn₂ does not exceed an absolute value of Δn₁,and therefore, a wavelength dependency of the term of the difference inrefractive index N(λ) in the equation (29) is increased in accordancewith an increase in a wavelength λ. Therefore, λ in the denominator inthe equation (26) is effectively canceled by the term (λ), and thus awavelength dependency of phase amplitude is reduced and further thewavelength dependency of diffraction efficiency is decreased.

FIG. 33 shows wavelength dependencies of diffraction efficiency of thediffractive lens of the present embodiment and a known diffractive lens.In FIG. 33, a solid curve denotes the wavelength dependency ofdiffraction efficiency of the diffractive lens of the presentembodiment, and a broken curve represents the wavelength dependency ofdiffraction efficiency of the known diffractive lens. In the knowndiffractive lens, a brazed pattern is formed on a “sitop” substrate(optimizing wavelength is 510 nm). As can be seen from FIG. 15, in thediffractive lens according to the invention, the wavelength dependencyof diffraction efficiency is effectively reduced as compared with theknown diffractive lens.

As explained above, in the diffractive lens of the present embodiment,it is possible to obtain a high diffraction efficiency over a wholevisible wavelength range, and therefore a problem of flare and ghosthardly occur. The diffractive lens of the present embodiment may beadvantageously utilized in an image pick-up device such as camera.Furthermore, the first and fourth optical regions 135 and 138 are madeof plastic materials, and thus it is possible to manufacture the firstand second relief patterns 233 and 234 in a very simple manner. Sincethe second and third optical regions 136 and 137 are made of the UVcurable resin, the first and second relief patterns 233 and 234 formedon the surfaces of the first and fourth optical regions 135 and 138 canbe cemented by means of the UV curable resin constituting the second andthird optical regions 136 and 137. In this manner, the diffractiveoptical element having the reduced wavelength dependency of diffractionefficiency can be easily manufactured at a low cost.

FIG. 34 shows an eighth embodiment of the diffractive optical elementaccording to the fourth aspect of the invention. The embodiment is adouble-focus type diffractive lens. The layered structure of the presentembodiment is same as that of the sixth embodiment shown in FIG. 26.That is to say, a first optical region 131 is made of an optical glassLaL14 (nd=1.6968, υd=55.5) manufactured and sold by OHARA company,second and third optical regions 132 and 133 are made of a UV curableresin (nd=1.52, υd=52), and a fourth optical region 134 is made of apolycarbonate (nd=1.58, υd=30.5). A first relief pattern 235 is formedin a boundary surface between the first and second optical regions 131and 132, and a second relief pattern 236 is formed in a boundary surfacebetween the third and fourth optical regions 133 and 134. The first andsecond relief patterns 235 and 236 have an identical pitch distributionand are aligned in a direction of an optical axis in such a manner thattop portions of the first relief pattern 235 are brought into contactwith bottom portions of the second relief pattern 236.

The pitch distribution of the first and second relief patterns 235 and236 is optimized to have a given lens function. These relief patterns235 and 236 are formed into a cross sectional shape having rectangulardepressions and protrusions, a ratio of the depressions to theprotrusions is equal to unity. A depth of the depressions andprotrusions is optimized such that the diffraction efficiency offirst-order becomes maximum at a wavelength of λ=600 nm. In the presentembodiment, a depth d₁ of the first relief pattern 235 is set to 4.02 μmand a depth d₂ of the second relief pattern 236 is set to 7.03 μm. Then,the parameter α defined by a ratio of d₂/d₁ as shown in the equation(27) is equal to about 1.75. Outermost surfaces 321 and 322 of thediffractive optical element are formed to be flat and anti-reflectingcoatings are applied on these outermost surfaces.

In the present embodiment, when diffraction efficiencies of ±1-orderbecome maximum, the phase amplitude corresponding to the equation (26)may be expressed as follows:

$\begin{matrix}{{a(\lambda)} = {\frac{m_{0}}{2} \cdot \frac{\left\{ {{n_{1}(\lambda)} - {n_{2}(\lambda)}} \right\} + {\alpha\left\{ {{n_{3}(\lambda)} - {n_{4}(\lambda)}} \right\}}}{\left\{ {{n_{1}\left( \lambda_{0} \right)} - {n_{2}\left( \lambda_{0} \right)}} \right\} + {\alpha\left\{ {{n_{3}\left( \lambda_{0} \right)} - {n_{4}\left( \lambda_{0} \right)}} \right\}}} \cdot \frac{\lambda_{0}}{\lambda}}} & (35)\end{matrix}$

Then, the diffraction efficiency of m-order ηm may be represented by thefollowing equation (36):

$\begin{matrix}{\eta_{m} = \left\{ {{\frac{\sin\left( {m\;{\pi/2}} \right)}{m\;{\pi/2}} \cdot \cos}\;{\pi\left( {a + \frac{m}{2}} \right)}} \right\}^{2}} & (36)\end{matrix}$

The phase amplitude denoted by the equation (36) may be obtained bydiving the right hand term in the equation (26) by two. This means thatalso in the present embodiment, the wavelength dependency of phaseamplitude can be reduced by the same mechanism as that of the sixthembodiment.

FIG. 35 shows the wavelength dependency of diffraction efficiency of±1-order in the diffractive lens of the present embodiment (solid curve)and a wavelength dependency of a known diffractive lens in which arectangular phase grating is formed in a LAL14 substrate (broken curve).From the graph shown in FIG. 35, it is apparent that according to theinvention, the wavelength dependency of diffraction efficiency iscorrected very well as compared with the known diffractive opticalelement. The wavelength dependency of diffraction efficiency iscorrected over a whole visible wavelength range, and thus thediffractive lens of the present embodiment is preferably used as adouble-focus optical system for a visible wavelength range.

In the above explained six and seventh embodiments, the first and secondrelief patterns are formed to have the sawtooth cross sectionalconfiguration, and in the eighth embodiment, the first and second reliefpatterns are shaped to have the rectangular cross sectionalconfiguration, However, according to the invention, the cross sectionalconfiguration of the first and second relief patterns is not restrictedto these configurations, but any other configuration may be used.

FIG. 36 illustrates a ninth embodiment of the diffractive opticalelement according to the invention. the diffractive optical element ofthe present embodiment is formed as a transmission type diffractivelens. A first optical region 141 is made of an acrylic resin (nd=1.49,υd=57.7), and second and third optical regions 142 and 143 are made of apolycarbonate (nd=1.58, υd=30.5). These optical regions are stacked oneon the other. In the present embodiment, a fourth optical region isconstituted by the air surrounding the optical element. A first reliefpattern 241 is formed in a boundary surface between the first and secondoptical regions 141 and 142, and a second relief pattern 242 is formedin a boundary surface between the third optical region 143 and the air,i.e. the fourth optical region. That is to say, the second reliefpattern 242 is formed in a free surface of the third optical region 143.The first and second relief patterns 241 and 242 have an identical pitchdistribution and are aligned in a direction of an optical axis in such amanner that top and bottom portions of the first relief pattern arealigned with bottom and top portions of the second relief pattern viewedin the direction of the optical axis. In the present embodiment, the topportions of the first relief pattern 241 are brought into contact withthe bottom portions of the second relief pattern 242.

The pitch distribution of the first and second relief patterns 241 and242 is optimized to have a given light collimating function and a depthof recesses of a sawtooth cross sectional configuration is optimizedsuch that the diffraction efficiency of first-order becomes maximum at awavelength of λ=550 nm. In the present embodiment, a depth d₁ of thefirst relief pattern 241 is 15.6 μm and a depth d₂ of the second reliefpattern 242 is 3.34 μm. Therefore, the parameter α defined by a ratio ofd₂/d₁ is equal to about 0.22.

In the present embodiment, the fourth optical region is formed by theatmosphere surrounding the optical element, practically the air having arefractive index of 1. Therefore, the wavelength dependency ofdiffraction efficiency can be corrected by the same function as theprevious embodiments. Particularly, the fourth optical region has a verylow refractive index, and thus a difference in refractive index betweenthe third optical region 143 and the fourth optical region Δn₂ issufficiently large. Then, a depth of the second relief pattern 242 canbe relatively small. Therefore, the diffractive lens of the presentinvention can be thin, and the pitch of the relief patterns can besmall.

In FIG. 37, a solid curve denotes the wavelength dependency ofdiffraction efficiency of the diffractive lens of the present embodimentand a broken curve shows a wavelength dependency of diffractionefficiency of a known diffractive lens in which a brazed pattern isformed in an acrylic resin substrate (optimizing wavelength λ=510 nm).From the graph shown in FIG. 37, it is apparent that also in the presentembodiment, the wavelength dependency of diffraction efficiency iscorrected efficiently as compared with the known diffractive lens.

FIG. 38 illustrates a tenth embodiment of the diffractive opticalelement according to the fourth aspect of the invention. The diffractiveoptical element of the present embodiment is formed as a transmissiontype diffractive lens. In the embodiments according to the fourth aspectof the invention shown in FIGS. 26, 30, 31, 34 and 36, the second andthird optical regions are made of the same optical material. However, inthe present embodiment, these optical regions are made different opticalmaterials. A first optical region 151 is made of an LaL14 optical glassmanufactured by OHARA (nd=1.6968, υd=55.5), a second optical region 152is made of a polycarbonate (nd=1.58, υd=30.5), and a third opticalregion 153 is made of a UV curable resin (nd=1.52, υd=52). Further, thefourth optical region is constituted by the air surrounding thediffractive optical element. A first relief pattern 251 is formed in aboundary surface between the first and second optical regions 151 and152, and a second relief pattern 252 is formed in a boundary surfacebetween the third optical region 153 and the air, i.e. the fourthoptical region. The first and second relief patterns 251 and 252 have anidentical pitch distribution and are aligned in a direction of anoptical axis in such a manner that top and bottom portions of the firstrelief pattern are aligned with bottom and top portions of the secondrelief pattern viewed in the direction of the optical axis. In thepresent embodiment, the top portions of the first relief pattern 251 arebrought into contact with the bottom portions of the second reliefpattern 252.

The pitch distribution of the first and second relief patterns 251 and252 is optimized to have a given lens function and a depth of recessesof a sawtooth cross sectional configuration is optimized such that thediffraction efficiency of first-order becomes maximum at a wavelength ofλ=550 nm. In the present embodiment, a depth d₁ of the first reliefpattern 251 is −12.98 μm and a depth d₂ of the second relief pattern 252is 1.82 μm. Therefore, the parameter α defined by a ratio of d₂/d₁ isequal to about −0.14.

Also in the present embodiment, a difference in refractive index betweenthe first optical region 151 and the second optical region 152 Δn₁ and adifference in refractive index between the third optical region 153 andthe air Δn₂ are given by the equations (23) and (24), respectively.Therefore, the wavelength dependency of diffraction efficiency can becorrected by the same function as the previous embodiments. That is tosay, in the present embodiment, a sign of the parameter α is selectedsuch that the phase shift functions of the first and second reliefpatterns 251 and 252 are mutually opposed, and thus a sign of Δn₂ whoseabsolute value decreases in accordance with an increase in wavelength isopposite to a sign of Δn₁. Therefore, a wavelength dependency of theterm of the difference in refractive index N(λ) is increased inaccordance with an increase in wavelength. Then, λ in the denominator ofthe equation (26) is well canceled by the term including the differencein refractive index. Therefore, the wavelength dependency of phaseamplitude is decreased, and thus the wavelength dependency ofdiffraction efficiency is also decreased.

Moreover, in the present embodiment, the second and third opticalregions 152 and 153 are made of different optical materials, and thus itis possible to set the differences in refractive index Δn₁ and Δn₂suitable values independently from each other. Therefore, a suitablecombination of the optical materials can be easily selected. Further,the fourth optical region is made of the air, the depth of the first andsecond relief patterns 251 and 252 can be thinner than that of theembodiment shown in FIG. 26, and thus it is possible to realize a thintype diffractive lens.

In FIG. 40, a solid curve denotes the wavelength dependency ofdiffraction efficiency of the diffractive lens of the present embodimentand a broken curve shows a wavelength dependency of diffractionefficiency of a known diffractive lens in which a brazed pattern isformed in a LaL14 glass substrate (optimizing wavelength λ=510 nm). Fromthe graph shown in FIG. 40, it is apparent that also in the presentembodiment, the wavelength dependency of diffraction efficiency iscorrected very well upon compared with the known diffractive lens.

In the above explained sixth to tenth embodiments according to thefourth aspect of the invention, a distance between the first and secondrelief patterns d=d₃+d₄ is set to zero. However, according to theinvention, the phase amplitude of the diffractive optical element is notbasically dependent upon said distance as can be read from the equations(26) and (28).

FIG. 39 shows a modification of the tenth embodiment illustrated in FIG.38. In the present embodiment, the second optical regions 152 and thirdoptical region 153 are cemented to each other by means of an adhesivelayer 160. Also in the present modified embodiment, the phase amplitudesare given by the equations (26) and (28), and therefore the wavelengthdependency of diffraction efficiency can be decreased just like as thetenth embodiment.

It should be noted that the distance between the first and second reliefpatterns becomes important when a thickness of the diffractive opticalelement is taken into account. That is to say, it is preferable toselect the distance d such that Q<1 is satisfied, wherein Q is theparameter defined by the equation (30). When this condition issatisfied, the diffractive optical element can be considered to be athin type. In this case, the first and second relief patterns areconsidered to be substantially brought into contact with each other.

In the structure depicted in FIG. 39, a thickness of the adhesive layer160 is d₅ and a refractive index of the adhesive layer is n₅, the depthD and average refractive index n₀′ corresponding to the equations (31)and (32), respectively may be expressed as follows:D=d ₁ +d ₂ +d ₃ +d ₄ +d ₅  (37)

$\begin{matrix}{n_{0} = \frac{{d_{1}\frac{n_{1} + n_{2}}{2}} + {d_{2}\frac{n_{3} + n_{4}}{2}} + {d_{3}n_{2}} + {d_{4}n_{3}} + {d_{5}n_{5}}}{d_{1} + d_{2} + d_{3} + d_{4} + d_{5}}} & (38)\end{matrix}$

The depth D of the periodic structure may be expressed by a simple sumof thicknesses of the optical regions in which the first and secondrelief patterns are formed and the adhesive layer. Further, the averagerefractive index n₀′ may be denoted by an average value of refractiveindices of these regions.

As explained above in detail, according to the invention, by suitablyselecting a ratio of the depth of the second relief pattern to the depthof the second relief pattern α(=d₂/d₁), the wavelength dependency ofphase amplitude can be controlled independently from opticalcharacteristics of optical materials such as substrate. Therefore, thewavelength dependency of diffraction efficiency can be optimized forrespective applications, and a suitable combination of optical materialsfor reducing the wavelength dependency of diffraction efficiency can beeasily found. In this manner, the diffractive optical element having thereduced wavelength dependency of diffraction efficiency can be realizedat a low cost.

1. An image pick-up optical system comprising: a refractive type lens;and a diffractive optical element having a diffraction efficiency for aparticular diffraction order more than 0.8 for a wavelength range from0.4 μm to 0.7 μm, said diffractive optical element and refractive typelens being arranged along an optical axis of the image pick-up opticalsystem.
 2. The image pick-up optical system according to claim 1,wherein said diffractive optical element includes first, second andthird optical regions which are arranged successively viewed in adirection of the optical axis, a first relief pattern formed in a firstboundary surface between the first and second optical regions, and asecond relief pattern formed in a second boundary surface between thesecond and third optical regions.
 3. The image pick-up optical systemaccording to claim 2, wherein a depth d1 of the first relief pattern anda depth d2 of the second relief pattern are smaller than 10 μm.
 4. Theimage pick-up optical system according to claim 2, wherein a depth d1 ofthe first relief pattern is smaller than a depth d2 of the second reliefpattern.
 5. The image pick-up optical system according to claim 2,wherein an Abbe's number u1 of the first optical region is larger thanan Abbe's number u3 of the third optical region.
 6. The image pick-upoptical system according to claim 2, wherein each of said first andsecond relief patterns has a rectangular cross sectional configurationhaving top and bottom portions.
 7. The image pick-up optical systemaccording to claim 6, wherein each of said first, second and thirdoptical regions has a substantially constant refractive index.
 8. Theimage pick-up optical system according to claim 7, wherein a refractiveindex n1 of the first optical region is different from a refractiveindex n3 of the third optical region and the refractive indices n1 andn3 are larger than a refractive index n2 of the second optical region.9. The image pick-up optical system according to claim 6, wherein saidfirst and second relief patterns have an identical pitch distributionand are arranged in a direction of the optical axis such that the topand bottom portions of the first relief pattern are aligned with thebottom and top portions of the second relief pattern, respectively. 10.The image pick-up optical system according to claim 9, wherein saidfirst and second relief patterns are arranged such that a space isformed between the top portions of the first relief pattern and thebottom portions of the second relief pattern viewed in the direction ofthe optical axis.
 11. The image pick-up optical system according toclaim 9, wherein said first and second relief patterns are arranged suchthat the top portions of the first relief pattern are contacted with thebottom portions of the second relief pattern viewed in the direction ofthe optical axis.
 12. An image pick-up optical system comprising: arefractive type lens; and a diffractive optical element having adiffraction efficiency more than 0.9 for a wavelength range from 0.4 μmto 0.7 μm, said diffractive optical element and refractive type lensbeing arranged along an optical axis of the image pick-up opticalsystem, and said diffractive optical element includes first, second andthird optical regions which are arranged successively viewed in adirection of the optical axis, a first relief pattern formed in a firstboundary surface between the first and second optical regions, and asecond relief pattern famed in a second boundary surface between thesecond and third optical regions, wherein each of said first and secondrelief patterns is constructed as a blazed grating.
 13. The imagepick-up optical system according to claim 12, wherein a surface of thefirst optical region opposed to the surface in which the first reliefpattern is fanned is curved and a surface of the third optical regionopposed to the surface in which the second relief pattern is formed iscurved.
 14. The image pick-up optical system according to claim 12,wherein each of said first and second relief patterns has a sawtoothcross sectional configuration including sharply raining sides and gentlyraising sides.
 15. The image pick-up optical system according to claim14, wherein said first and second relief patterns have an identicalpitch distribution and are arranged in a direction of the optical axissuch that top and bottom portions of the first relief pattern arealigned with top and bottom portions of the second relief pattern,respectively.
 16. The image pick-up optical system according to claim15, wherein said first and second relief patterns are arranged such thata space is formed between the top portions of the first relief patternand the bottom portions of the second relief pattern viewed in thedirection of the optical axis.
 17. The image pick-up optical systemaccording to claim 15, wherein said first and second relief patterns arearranged such that the top portions of the first relief pattern arecontacted with the bottom portions of the second relief pattern viewedin the direction of the optical axis.
 18. The image pick-up opticalsystem according to claim 15, wherein said first and second reliefpatterns are arranged such that the top portions of the first reliefpattern are extended up to points situating between the bottom portionsand the top portions of the second relief pattern viewed in thedirection of the optical axis.
 19. The image pick-up optical systemaccording to claim 14, wherein said first and second relief patternshave an identical pitch distribution and are aligned in a direction ofthe optical axis such that top and bottom portions of the first reliefpattern are aligned with bottom and top portions of the second reliefpattern, respectively.
 20. The image pick-up optical system according toclaim 19, wherein said first and second relief patterns are arrangedsuch that a apace is formed between the top portions of the first reliefpattern and the bottom portions of the second relief pattern viewed inthe direction of the optical axis.
 21. The image pick-up optical systemaccording to claim 19, wherein said first and second relief patterns arearranged such that the top portions of the first relief pattern arecontacted with the bottom portions of the second relief pattern viewedin the direction of the optical axis.
 22. The image pick-up opticalsystem according to claim 14, wherein each of said first, second andthird optical regions has a substantially constant refractive index. 23.The image pick-up optical system according to claim 22, wherein arefractive index n1 of the first optical region is different from arefractive index n3 of the third optical region and the refractiveindices n1 and n3 are larger than a refractive index n2 of the secondoptical region.
 24. A camera comprising an image pick-up optical systemaccording to any one of claims 1-11.
 25. A camera comprising: an imagepick-up optical system for forming an image of an object to bepicked-up; and an image pick-up element receiving the image of theobject and generating an image signal: wherein said image pick-upoptical system comprising: a refractive type lens; and a diffractiveoptical element having a diffraction efficiency for a particulardiffraction order more than 0.8 for a wavelength range from 0.4 μm to0.7 μm, said diffractive optical element and refractive type lens beingarranged along an optical axis of the image pick-up optical system. 26.The camera according to claim 25, wherein said diffractive opticalelement includes first, second and third optical regions which arearranged successively viewed in a direction of the optical axis, a firstrelief pattern formed in a first boundary surface between the first andsecond optical regions, and a second relief pattern formed in a secondboundary surface between the second and third optical regions.
 27. Thecamera according to claim 26, wherein a depth d1 of the first reliefpattern and a depth d2 of the second relief pattern are smaller than 10μm.
 28. The camera according to claim 26, wherein a depth d1 of thefirst relief pattern is smaller than a depth d2 of the second reliefpattern.
 29. The camera according to claim 26, wherein an Abbe's numberu1 of the first optical region is larger than an Abbe's number u3 of thethird optical region.
 30. The camera according to claim 26, wherein eachof said first and second relief patterns has a rectangular crosssectional configuration having top and bottom portions.
 31. The cameraaccording to claim 30, wherein each of said first, second and thirdoptical regions has a substantially constant refractive index.
 32. Thecamera according to claim 31, wherein a refractive index n1 of the firstoptical region is different from a refractive index n3 of the thirdoptical region and the refractive indices n1 and n3 are larger than arefractive index n2 of the second optical region.
 33. The cameraaccording to claim 30, wherein said first and second relief patternshave an identical pitch distribution and are arranged in a direction ofthe optical axis such that the top and bottom portions of the firstrelief pattern are aligned with the bottom and top portions of thesecond relief pattern, respectively.
 34. The camera according to claim33, wherein said first and second relief patterns are arranged such thata space is formed between the top portions of the first relief patternand the bottom portions of the second relief pattern viewed in thedirection of the optical axis.
 35. The camera according to claim 33,wherein said first and second relief patterns are arranged such that thetop portions of the first relief pattern are contacted with the bottomportions of the second relief pattern viewed in the direction of theoptical axis.
 36. A camera comprising: an image pick-up optical systemfor forming an image of an object to be picked-up; and an image pick-upelement receiving the image of the object and generating an imagesignal: wherein said image pick-up optical system comprising: arefractive type lens; and a diffractive optical element having adiffraction efficiency more than 0.8 for a wavelength range from 0.4 μmto 0.7 μm, said diffractive optical element and refractive type lensbeing arranged along an optical axis of the image pick-up opticalsystem, and said diffractive optical element includes first, second andthird optical regions which are arranged successively viewed in adirection of the optical axis, a first relief pattern formed in a firstboundary surface between the first and second optical regions, and asecond relief pattern formed in a second boundary surface between thesecond and third optical regions, and each of said first and secondrelief patterns is constructed as a blazed grating.
 37. The cameraaccording to claim 36, wherein a surface of the first optical regionopposed to the surface in which the first relief pattern is formed iscurved and a surface of the third optical region opposed to the surfacein which the second relief pattern is formed is curved.
 38. The cameraaccording to claim 36, wherein each of said first and second reliefpatterns has a sawtooth cross sectional configuration including sharplyraising sides and gently raising sides.
 39. The camera according toclaim 38, wherein each of said first, second and third optical regionshas a substantially constant refractive index.
 40. The camera accordingto claim 39, wherein a refractive index n1 of the first optical regionis different from a refractive index n3 of the third optical region andthe refractive indices n1 and n3 are larger than a refractive index n2of the second optical region.
 41. The camera according to claim 38,wherein said first and second relief patterns have an identical pitchdistribution and are arranged in a direction of the optical axis suchthat top and bottom portions of the first relief pattern are alignedwith top and bottom portions of the second relief pattern, respectively.42. The camera according to claim 41, wherein said first and secondrelief patterns are arranged such that a apace is formed between the topportions of the first relief pattern and the bottom portions of thesecond relief pattern viewed in the direction of the optical axis. 43.The camera according to claim 41, wherein said first and second reliefpatterns are arranged such that the top portions of the first reliefpattern are contacted with the bottom portions of the second reliefpattern viewed in the direction of the optical axis.
 44. The cameraaccording to claim 41, wherein said first and second relief patterns arearranged such that the top portions of the first relief pattern areextended up to points situating between the bottom portions and the topportions of the second relief pattern viewed in the direction of theoptical axis.
 45. The camera according to claim 38, wherein said firstand second relief patterns have an identical pitch distribution and arealigned in a direction of the optical axis such that top and bottomportions of the first relief pattern are aligned with bottom and topportions of the second relief pattern, respectively.
 46. The cameraaccording to claim 45, wherein said first and second relief patterns arearranged such that a space is formed between the top portions of thefirst relief pattern and the bottom portions of the second reliefpattern viewed in the direction of the optical axis.
 47. The cameraaccording to claim 45, wherein said first and second relief patterns arearranged such that the top portions of the first relief pattern arecontacted with the bottom portions of the second relief pattern viewedin the direction of the optical axis.
 48. A view finder of a cameracomprising: a refractive type lens; and a diffractive optical elementhaving a diffraction efficiency for a particular diffraction order morethan 0.8 for a wavelength range from 0.4 μm to 0.7 μm, said diffractiveoptical element and refractive type lens being arranged along an opticalaxis of the view finder.
 49. The view finder according to claim 48,wherein said diffractive optical element includes first, second andthird optical regions which are arranged successively viewed in adirection of the optical axis, a first relief pattern formed in a firstboundary surface between the first and second optical regions, and asecond relief pattern formed in a second boundary surface between thesecond and third optical regions.
 50. The view finder according to claim49, wherein a depth d1 of the first relief pattern and a depth d2 of thesecond relief pattern are smaller than 10 μm.
 51. The view finderaccording to claim 49, wherein a depth d1 of the first relief pattern issmaller than a depth d2 of the second relief pattern.
 52. The viewfinder according to claim 49, wherein an Abbe's number u1 of the firstoptical region is larger than are Abbe's number u3 of the third opticalregion.
 53. The view finder according to claim 49, wherein each of saidfirst and second relief patterns is constructed as a blazed grating. 54.The view finder according to claim 53, wherein each of said first andsecond relief patterns has a sawtooth cross Sectional configurationincluding sharply raising sides and gently raising sides.
 55. The viewfinder according to claim 54, wherein each of said first, second andthird optical regions has a substantially constant refractive index. 56.The view finder according to claim 55, wherein a refractive index n1 ofthe first optical region is different from a refractive index n3 of thethird optical region and the refractive indices n1 and n3 are largerthan a refractive index n2 of the second optical region.
 57. The viewfinder according to claim 54, wherein said first and second reliefpatterns have an identical pitch distribution and are arranged in adirection of the optical axis such that top and bottom portions of thefirst relief pattern are aligned with top and bottom portions of thesecond relief pattern, respectively.
 58. The view finder according toclaim 57, wherein said first and second relief patterns are arrangedsuch that a space is formed between the top portions of the first reliefpattern and the bottom portions of the second relief pattern viewed inthe direction of the optical axis.
 59. The view finder according toclaim 57, wherein said first and second relief patterns are arrangedsuch that the top portions of the first relief pattern are contactedwith the bottom portions of the second relief pattern viewed in thedirection of the optical axis.
 60. The view finder according to claim57, wherein said first and second relief patterns are arranged such thatthe top portions of the first relief pattern are extended up to pointssituating between the bottom portions and the top portions of the secondrelief pattern viewed in the direction of the optical axis.
 61. The viewfinder according to claim 54, wherein said first and second reliefpatterns have an identical pitch distribution and are aligned in adirection of the optical axis such that top and bottom portions of thefirst relief pattern are aligned with bottom and top portions of thesecond relief pattern, respectively.
 62. The view finder according toclaim 61, wherein said first and second relief patterns are arrangedsuch that a space is formed between the top portions of the first reliefpattern and the bottom portions of the second relief pattern viewed inthe direction of the optical axis.
 63. The view finder according toclaim 61, wherein said first and second relief patterns are arrangedsuch that the top portions of the first relief pattern are contactedwith the bottom portions of the second relief pattern viewed in thedirection of the optical axis.
 64. The view finder according to claim53, wherein a surface of the first optical region opposed to the surfacein which the first relief pattern is formed is curved and a surface ofthe third optical region opposed to the surface in which the secondrelief pattern is formed is curved.
 65. The view finder according toclaim 49, wherein each of said first and second relief patterns has arectangular cross sectional configuration having top and bottomportions.
 66. The view finder according to claim 65, wherein each ofsaid first, second and third optical regions has a substantiallyconstant refractive index.
 67. The view finder according to claim 66,wherein a refractive index n1 of the first optical region is differentfrom a refractive index n3 of the third optical region and therefractive indices n1 and n3 are larger than a refractive index n2 ofthe second optical region.
 68. The view finder according to claim 65,wherein said first and second relief patterns have an identical pitchdistribution and are arranged in a direction of the optical axis suchthat the top and bottom portions of the first relief pattern are alignedwith the bottom and top portions of the second relief pattern,respectively.
 69. The view finder according to claim 68, wherein saidfirst and second relief patterns are arranged such that a space isformed between the top portions of the first relief pattern and thebottom portions of the second relief pattern viewed in the direction ofthe optical axis.
 70. The view finder according to claim 68, whereinsaid first and second relief patterns are arranged such that the topportions of the first relief pattern are contacted with the bottomportions of the second relief pattern viewed in the direction of theoptical axis.
 71. An image pick-up optical system comprising: arefractive type lens; and two diffractive optical surfaces, saiddiffractive optical surfaces and refractive type lens being arrangedalong an optical axis of the image pick-up optical system and saiddiffractive optical surfaces being arranged successively viewed in adirection of the optical axis; wherein each of the diffractive opticalsurfaces is formed as a relief pattern having a sawtooth cross sectionalconfiguration including sharply raising sides and gently raising sidesand said diffractive optical surfaces having a diffraction efficiencymore than 0.8 for a wavelength range from 0.4 μm to 0.7 μm.
 72. Theimage pick-up optical system according to claim 71, wherein said twodiffractive optical surfaces are fanned by a first relief pattern and asecond relief pattern, and a depth d1 of the first relief pattern and adepth d2 of the second relief pattern are smaller than 10 μm.
 73. Theimage pick-up optical system according to claim 71, wherein said twodiffractive optical surfaces are formed by a first relief pattern and asecond relief pattern, and a depth d1 of the first relief pattern issmaller than a depth d2 of the second relief pattern.
 74. The imagepick-up optical system according to claim 71, wherein said twodiffractive optical surfaces are formed by a first relief pattern and asecond relief pattern, and said first and second relief patterns have anidentical pitch distribution and are arranged in a direction of theoptical axis such that top and bottom portions of the first reliefpattern are aligned with top and bottom portions of the second reliefpattern, respectively.
 75. The image pick-up optical system according toclaim 74, wherein said two diffractive optical surfaces are formed by afirst relief pattern and a second relief pattern, and said first andsecond relief patterns are arranged such that the top portions of thefirst relief pattern are contacted with the bottom portions of thesecond relief pattern viewed in the direction of the optical axis. 76.The image pick-up optical system according to claim 71, wherein said twodiffractive optical surfaces are formed by a first relief pattern and asecond relief pattern, and said first and second relief patterns arearranged such that a space is formed between the top portions of thefirst relief pattern and the bottom portions of the second reliefpattern viewed in the direction of the optical axis.
 77. The imagepick-up optical system according to claim 71, wherein said twodiffractive optical surfaces are formed by a first relief pattern and asecond relief pattern, and said first and second relief patterns arearranged such that the top portions of the first relief pattern areextended up to points situating between the bottom portions and the topportions of the second relief pattern viewed in the direction of theoptical axis.
 78. The image pick-up optical system according to claim71, wherein said two diffractive optical surfaces are formed by a firstrelief pattern and a second relief pattern, and said first and secondrelief patterns have an identical pitch distribution and are aligned ina direction of the optical axis such that top and bottom portions of thefirst relief pattern are aligned with bottom and top portions of thesecond relief pattern, respectively.
 79. The image pick-up opticalsystem according to claim 78, wherein said two diffractive opticalsurfaces are formed by a first relief pattern and a second reliefpattern, and said first and second relief patterns are arranged suchthat the top portions of the first relief pattern are contacted with thebottom portions of the second relief pattern viewed in the direction ofthe optical axis.
 80. The image pick-up optical system according toclaim 71 comprising a non-diffractive optical surface arranged betweenthe two diffractive optical surfaces.
 81. The image pick-up opticalsystem according to claim 80, wherein said two diffractive opticalsurfaces are formed by a first relief pattern and a second reliefpattern, and a depth d1 of the first relief pattern and a depth d2 ofthe second relief pattern are smaller than 10 μm.
 82. The image pick-upoptical system according to claim 80, wherein said two diffractiveoptical surfaces are formed by a first relief pattern and a secondrelief pattern, and a depth d1 of the first relief pattern is smallerthan a depth d2 of the second relief pattern.
 83. The image pick-upoptical system according to claim 80, wherein said two diffractiveoptical surfaces are formed by a first relief pattern and a secondrelief pattern, and each of said first and second relief patterns isconstructed as a blazed grating.
 84. The image pick-up optical systemaccording to claim 80, wherein said two diffractive optical surfaces areformed by a first relief pattern and a second relief pattern, and saidfirst and second relief patterns have an identical pitch distributionand are arranged in a direction of the optical axis such that top andbottom portions of the first relief pattern are aligned with top andbottom portions of the second relief pattern, respectively.
 85. Theimage pick-up optical system according to claim 84, wherein said firstand second relief patterns are arranged such that a space is formedbetween the top portions of the first relief pattern and the bottomportions of the second relief pattern viewed in the direction of theoptical axis.
 86. The image pick-up optical system according to claim84, wherein said first and second relief patterns are arranged such thatthe top portions of the first relief pattern are contacted with thebottom portions of the second relief pattern viewed in the direction ofthe optical axis.
 87. The image pick-up optical system according toclaim 80, wherein said two diffractive optical surfaces are formed by afirst relief pattern and a second relief pattern, and said first andsecond relief patterns have an identical pitch distribution and arealigned in a direction of the optical axis such that top and bottomportions of the first relief pattern are aligned with bottom and topportions of the second relief pattern, respectively.
 88. The imagepick-up optical system according to claim 87, wherein said first andsecond relief patterns are arranged such that the top portions of thefirst relief pattern are contacted with the bottom portions of thesecond relief pattern viewed in the direction of the optical axis. 89.The image pick-up optical system according to claim 87, wherein arefractive index of an optical material situating between saidnon-diffractive optical surface and said second relief pattern is lowerthan a refractive index of an optical material situating on a that sideof said second relief pattern, which is opposite to said non-diffractiveoptical surface.
 90. The image pick-up optical system according to claim87, wherein said first relief pattern is formed on one surface of afirst thin layer the other surface of which is curved, and said secondrelief pattern is formed on one surface of a second thin layer, theother surface of which is curved in a same direction in which said othersurface of said first thin layer is curved.
 91. The image pick-upoptical system according to claim 80, wherein said two diffractiveoptical surfaces are formed by a first relief pattern and a secondrelief pattern, and a refractive index of a first optical materialsituating between said non-diffractive optical surface and said secondrelief pattern is lower than a refractive index of a second opticalmaterial situating on that side of said second relief pattern, which isopposite to said non-diffractive optical surface.
 92. The image pick-upoptical system according to claim 80, wherein said two diffractiveoptical surfaces are formed by a first relief pattern and a secondrelief pattern, said first relief pattern is formed on one surface of afirst thin layer the other surface of which is curved, and said secondrelief pattern is formed on one surface of a second thin layer the othersurface of which is curved in a same direction in which said othersurface of said first thin layer is curved.
 93. An image pick-up opticalsystem comprising: a refractive type lens; and two diffractive opticalsurfaces, said diffractive optical surfaces and refractive type lensbeing arranged along an optical axis of the image pick-up optical systemand said diffractive optical surfaces being arranged successively viewedin a direction of the optical axis; wherein each of the diffractiveoptical surfaces is formed as a relief pattern having a sawtooth crosssectional configuration including sharply raising sides and gentlyraising sides and said diffractive optical surfaces having a diffractionefficiency more than 0.8 for a wavelength range from 0.4 μm to 0.7 μm;wherein said two diffractive optical surfaces are formed by a firstrelief pattern and a second relief pattern, and each of said first andsecond relief patterns is constructed as a blazed grating.